Something arrives in a vegetable box, or looks interesting at the market, and then sits in the fridge until it is thrown away. The problem is not that it is difficult. It is that nobody tells you the general rule, so every new vegetable feels like it needs its own recipe.
Judge it by how hard and how watery it is
Hard and dense — roots, squashes, anything you cannot easily push a nail into — wants long heat: roasting, boiling, simmering in a soup or stew. Soft and watery — leaves, courgettes, mushrooms, tomatoes — wants short, high heat or no heat at all: frying, wilting, raw in a salad. Get this one distinction right and you will rarely be far wrong.
Roast first, decide later
If you have no idea, cut it into pieces of a similar size, toss with oil and salt, and roast at a high temperature until the edges colour. This works for almost every hard vegetable and many soft ones. It is also the most forgiving method: a few minutes too long makes it darker, not inedible.
Taste a small raw piece first
Bitter, sweet, starchy or peppery tells you what to do next. Bitter wants fat, salt and something sweet or acid against it. Starchy wants seasoning and more of it than you think. Peppery mellows with heat. Thirty seconds of tasting removes most of the guesswork, and it is a habit almost nobody has.
Cook half, keep half
Try a small amount the way that seems most likely, and keep the rest. If it works you have learned something; if not, you have not lost the whole vegetable. Write down what you did. Unfamiliar vegetables become familiar after exactly one successful attempt.
Hard and dense means long heat. Soft and watery means short heat. When in doubt, roast it, taste it raw first, and only cook half.