Almost every disappointing home-cooked meal is one ingredient away from being good, and the ingredient is almost never more salt. Taste before serving, work out which of five things is missing, and fix it while the pan is still hot.
Flat is the most common, and salt is rarely the answer
If it tastes like nothing in particular, it usually wants acid: a squeeze of lemon, a splash of vinegar, a spoon of yoghurt, a chopped pickle. Acid is what makes flavours separate from one another and become legible. Add a small amount, stir, taste again. If it is still flat after acid, then reach for salt — and if salt does nothing either, it wants fat: a knob of butter, a spoon of oil, some cheese.
Too salty
You cannot remove salt, so you dilute or balance it. Add more of the unsalted bulk — potato, rice, pasta water, a tin of tomatoes, more vegetables — or serve it with something plain that takes the edge off. A little acid and a little sweetness both make saltiness read as less harsh, which is why lemon on over-seasoned fish works. The old advice about dropping in a raw potato does not actually pull out salt.
Too sharp, too bitter, too sweet
Too sharp or too acidic: add fat or a pinch of sugar. Cream, butter, oil and yoghurt all round off an edge that vinegar or tomato has left. Too bitter — burnt garlic, over-browned onion, certain greens: fat and salt help, sweetness helps more, and acid makes it worse. If something is actually burnt, take out the burnt pieces before trying to fix the rest. Too sweet: acid and salt, in that order. A dish that has gone too sweet is usually rescued by lemon, not by adding something savoury.
Too heavy, too greasy
Something fresh and raw on top does more than any amount of stirring: herbs, raw onion, chopped tomato, a squeeze of citrus, a crunchy thing. If it is genuinely greasy, let it settle for a minute and spoon the fat off the top, or drop in a spoonful of cold water — the fat gathers and lifts more easily.
Taste on a spoon, not from the pan edge
Take a proper spoonful with a bit of everything in it. The edge of a pan is always more concentrated than the dish, and seasoning to that is how food ends up under-seasoned in the middle. And taste twice: once when you think it is done, and once after it has sat for two minutes. Flavours change as things cool slightly, and salt in particular reads stronger when the food is cooler.
Flat wants acid. Salty wants bulk. Bitter wants sweetness. Heavy wants something raw. Run those four checks with a spoon in your hand and very little needs to be thrown away or apologised for.