Vegetables are frequently cooked badly because they are treated as a single category. The relevant differences are structural and they determine what technique works.
Cell walls
Plant cells are held together by pectin, and cooking breaks it down, which is what softening actually is.
Pectin breaks down faster in alkaline conditions and slower in acid ones.
Which is why adding acid early to a dish with vegetables keeps them firm, and why beans cooked with tomatoes take considerably longer to soften.
It is also why a small amount of bicarbonate speeds up softening dramatically, and why too much produces mush.
Starch against water
The distinction that determines technique.
Starchy vegetables — potatoes, root vegetables, winter squash — absorb and hold, and they benefit from methods that develop their sugars.
High-water vegetables — courgettes, mushrooms, aubergines, greens — release liquid, which must be managed or the dish becomes wet.
Which is why mushrooms should go into a hot pan with space around them, so the released water evaporates rather than accumulating.
The salting question
Salting high-water vegetables in advance draws out moisture through osmosis.
Which concentrates flavour, improves browning and, in aubergines, was traditionally said to remove bitterness.
Modern varieties are far less bitter than older ones, so the bitterness argument largely no longer applies.
The moisture argument still does, particularly for anything that will be fried.
Green vegetables
Their colour comes from chlorophyll, which degrades with prolonged heat and with acid.
Which is why greens go from bright to olive, and why adding lemon early produces a duller result than adding it at the end.
Brief cooking preserves colour. Shocking in cold water stops the cooking and holds it.
Covering a pan traps acids released by the vegetables, which accelerates the colour loss, so cooking greens uncovered helps.
The brassica problem
Cabbage, sprouts, broccoli and their relatives contain sulphur compounds released by cooking.
Prolonged cooking releases considerably more, which is the source of the smell associated with badly cooked versions.
Which means fast cooking — roasting, high-heat sautéing, brief steaming — produces something entirely different from the boiled version many people remember.
Roasting also browns them, adding an entire flavour dimension that boiling cannot.
Umami without meat
The savoury character that vegetarian cooking is sometimes said to lack, and which is straightforwardly obtainable.
Mushrooms, particularly dried ones, are among the richest sources.
Tomatoes, especially concentrated as paste, contribute substantially.
Fermented ingredients — soy products, miso, aged cheeses where appropriate — contribute glutamates directly.
Seaweed is used for exactly this purpose in several cuisines.
And browning vegetables produces its own savoury compounds through the same reactions that operate on meat.
Fat
Many flavour compounds in vegetables are fat-soluble, and several nutrients are absorbed better with fat present.
Which means vegetables cooked without fat taste less and nourish less, and low-fat vegetable cookery is working against the chemistry.
The practical summary
Match the method to the water content. Use enough heat and enough space. Add acid late. Use fat. And brown things, because the same reactions apply regardless of what is being cooked.
Roasting
The method that most reliably improves vegetables people think they dislike.
High dry heat drives off water, concentrating flavour, and produces browning across the surface.
Which converts the vegetal character into something sweeter and more savoury.
Space on the tray matters — crowded vegetables steam, which is the most common reason roasting disappoints.
A genuinely hot oven and a single layer are the two requirements.
Blanching
Brief cooking in boiling water followed by cold water.
Which sets colour, softens partially, and deactivates enzymes that cause deterioration.
It is why vegetables intended for freezing are blanched first, and why prepared vegetables in a professional kitchen hold their colour.
Seasonality
Vegetables at their season are cheaper, better and require less doing to them.
Which is a practical argument rather than a romantic one, and it is why the same recipe produces very different results at different times of year.
Cooking what is good rather than what a recipe specifies is the habit that improves results most, and it requires shopping before deciding what to cook.
Frozen vegetables
Frozen soon after harvest, which frequently means better nutritional retention than fresh produce that has travelled and sat.
The texture is softer after thawing, which suits cooked applications and not raw ones.
Which makes them a genuinely good option rather than a compromise, and they are considerably cheaper.
Peel
Much of the fibre and several nutrients concentrate near the skin.
Which means peeling removes a meaningful portion, and for most vegetables a thorough wash is sufficient.
Cooking water
Water-soluble nutrients leach into cooking water, which is why steaming retains more than boiling.
Where boiling is used, keeping the water for a soup or sauce recovers what would otherwise be poured away.
The differences are modest and worth knowing.