I like to think of these food replacements as synonyms, of sorts. Synonyms have basically the same meaning, but they often have slight differences in their nuances. Hot and sweltering are synonyms, for example, but sweltering brings to mind sweat dripping down your face but wouldn't be used to describe a pot of soup, for example. Hot, on the other hand, has a more versatile usage, but it doesn't bring the same association to mind that sweltering does.
When it comes to replacing food items in recipes, you'll need to use your senses and noggin to figure out what words can best be used to describe the food you'd like to replace, and then find another food that can be described using those same words. You'll want to think about its taste and texture (most important) in addition to smell and looks (less important, but still relevant). When you replace a food with its "food synonym", you'll be altering the end result, but in almost every case the end result will be just as good, if not better, than the originally intended outcome.
This list overlaps somewhat with my Cheapskate Ingredient Substitution list, but is in no way a replica.
Sweet Potato Description: How shall I describe thee, sweet potato, so divine? You're expensive round these parts, but I miss your creamy, sweet, and orange interior. You're good in soups, in pies, and stews. Oh sweet potato, I love you!
Sweet Potato "Synonyms": Carrots, Pumpkin, and Butternut squash all can be described as orange, sweet, and creamy, so can replace sweet potatoes as well as each other in most foods.
Note that these all have different baking times, so you can't simply replace in a recipe with no changes. If a recipe calls for a baked or microwaved sweet potato, you can use a baked/microwaved pumpkin or butternut squash in its place, but the carrot will need to be boiled to have the same consistency as a baked sweet potato. Also note that sweet potatoes fall apart when boiled, so if a recipe calls for sweet potatoes cooked in liquids and you're replacing it with butternut squash or carrots, you may want to mash or blend the carrots somewhat so their flavor spreads throughout the dish as a sweet potato's would. Pumpkin, however, falls apart as a sweet potato would, so no changes need to be made.
I never buy sweet potatoes anymore and find these replacements to be quite adequate and cheaper too!
Specialty Mushroom Description: There are so many types of specialty mushrooms, from Beech mushrooms to Shitake mushrooms to Porcini mushrooms to Portabello mushrooms to King oyster mushrooms, that I couldn't possibly name them all. Each mushroom has a slightly varying taste, but have the same essential description- meaty, earthy, semi crisp when raw and soft and slug textured when cooked. (I happen to love mushrooms, but my husband's description of them being slug textured has stuck out in my mind as being pretty accurate nonetheless, which is why I used it.)
Specialty Mushroom "Synonyms": Unless using mushrooms in a way that their shape greatly affects their usage (such as in stuffed or grilled mushrooms), almost all mushrooms are interchangeable in recipes with the simple cheaper standard button mushroom, sold everywhere, or with other specialty mushrooms.
Bell Pepper Description. In every place I've lived so far, colorful peppers have been quite expensive. Red, orange, and yellow bell peppers can be twice or even triple the price of green peppers. The yellow, orange, and red peppers are all sweet and slightly sour, with the red peppers being the most tart. Green peppers taste similarly to the colorful peppers, but without the sweet. Crunchy when raw, and and soft when cooked, their skin gets more bitter when cooked.
Bell Pepper "Synonyms". Although each pepper has a slightly different taste with dark green peppers being the most drastically different, peppers are nearly all interchangable in recipes. Keep in mind though that cooked green peppers are the most bitter and lack the sweetness, so you may want to avoid using it in recipes that need that sweetness, or just add a drop of sugar to the recipe to account for the lack of sweetness. In my experience, the light green peppers end up being the best tasting for the least amount, so I usually use them as a replacement for the more colorful peppers.
Summer Squash Description. This vegetable has a soft skin and a semi sweet flesh and edible seeds that is crisp when raw and softens once cooked.
Summer Squash "Synonyms". There are many types of summer squash, from Crookneck squash to Pattypan squash to many types of Zucchini and they can all replace each other in recipes without changing the taste much. You can also use Cucumber as a replacement in recipes, but while this has a slightly different taste, it isn't different enough to change the taste of a dish.
Shallot Description. This flavorful plant imparts a delightful flavor on food. It is sharp raw, but its flavor mellows out once cooked. Its flavor is a cross between garlic and onion.
Shallot "Synonyms". Shallots can be replaced with Onions or Garlic, and the best replacement is a combination of those two.
Nappa Cabbage Description. This vegetable is crispy and mildly sweet and delicious, and has a remarkably similar taste to both regular green cabbage and purple cabbage.
Nappa Cabbage "Synonyms". You can easily replace nappa cabbage in most recipes with either Green cabbage or Purple cabbage. Note that regular cabbage will take a little longer to cook, so keep that in mind when using it in a recipe.
Spinach Description. To be perfectly honest, the taste of spinach is hard to describe, other than that it is earthy and slightly bitter. When fresh, it is crispy like lettuce; when cooked, it drastically reduces in volume and becomes mushy.
Spinach "Synonyms". Most greens are similar enough in taste, texture, and looks that they can replace each other in recipes. Because Swiss chard is the cheapest green where I live, that is usually my spinach synonym of choice. Other greens like Mustard greens, Collard greens, Turnip greens, Kale, or even Romaine lettuce can be used in place of spinach or of each other in recipes.
Note: If you replace spinach with another green, you may want to cut out the thick rib in the middle of the green and use it in another recipe so that it keeps the same texture as spinach.
Scallions Description. Also known as green onions or spring onions, this plant from the onion family has a mildly sharp oniony taste and is used in recipes where an oniony taste is desired but regular white onions would be too overpowering.
Scallions Synonyms. Scallions nearly identical tastes to both chives and onion greens and can be used interchangably in recipes with barely making even the slightest difference in taste. If neither of these options are available, you can use a tiny piece of raw onion (preferably purple) blended up or chopped into tiny pieces to impart a small amount of oniony taste.
Part 2 of my "Vegetable Synonyms" post will be coming in the future. For now, I'll leave you with this partial list to digest.
Do you ever use replacements for vegetables in recipes? Why do you do that? Because of cost? Taste preference? Availability? What is your most frequently used vegetable "synonym"? And by the way, what do you think of the word "synonym" being used to describe this type of vegetable replacement- does it make sense or just sound silly?