Mixed Vegetables are a mixture of at least two different vegetables. They are peeled, if necessary, and prepared in bite-sized pieces if they are large vegetables.
Though there are recipes for mixed-vegetable dishes, and there’s no doubting that these are indeed “mixed vegetables”, the phrase “Mixed Vegetables” now means to most people a commercial preparation, either canned or frozen.
The vegetables in commercial “Mixed Vegetables” used to be just peas and corn, or peas and carrots (with the carrots being diced to match the peas in size.) There are now many different mixtures: mixtures appropriate for stir fries, for Italian pasta dishes, etc. In Britain, you can buy bags of frozen mixed stewing vegetables appropriate for stews which include root vegetables such as swede, carrots, parsnip, etc.
Frozen Mixed Vegetables are anathema to many purists, owing to the mistaken notion that the “fresh” vegetables on their supermarket shelves are fresher than the frozen ones. They haven’t even been appeased by the newer moniker being applied to Mixed Vegetables by fancier packaging: Vegetable Medley.
When you are buying Mixed Vegetables, whether canned or frozen, you lose some flexibility. For example, picture a bag of frozen mixed peas and carrots versus two separate bags, one of peas and one of carrots. If you wanted to toss a handful of carrots into a soup, but not peas, with the bag of Mixed Vegetables you can’t, unless you’re willing to take the time to pick out all the peas. With the two separate bags, you can both use them separately as needed, and mix and match when you feel like it.
The carrots in frozen mixed peas and carrots never seem to get cooked enough, without boiling the peas into a mush.