2.) Stop eating over-flavoured foods. Doritos etc. have a ridiculous amount of flavouring; if you eat foods like this consistently then you'll desensitise your palette to the subtler but still absolutely lovely tastes of vegetables, in the same way that staring into a spotlight makes it difficult to appreciate the aesthetic qualities of twilight. You can't re-sensitise your palette by simply cutting down on hyperflavoured food; you need to eliminate it almost completely. After a while, you'll discover that brocoli and green beans are wonderful, and Doritos are absolutely brutal on the tongue.
Roasting helps add flavor. (Maillard reaction.) So do spices/herbs.Don't overcook. (Also butter, of course, but I assume the goal here is to keep it healthy.)
Roasting definitely helps, and I don't worry too much about how much olive oil I coat vegetables with.
For example, I'll roast some cauliflower for 45 minutes, and then I can eat it like that or mash it like potatoes (tastes way better than mashed potatoes to me).
String beans I'll coat with olive oil and chipotle powder and that tastes great.
I think the real question is: how are they unpalatable now?
I ask because taste bud response varies; one common reference is the supertaster-taster-nontaster continuum, which is focused on perception of bitter component found in some vegetables.
If you're a supertaster, you might not be able to do anything to make them completely acceptable to you; they're always going to be a bit over-bitter. Hate to say it, but cheese, butter, and gentle use of honey may be your best bets.
On the other hand, if you're a taster, the suggestions here will help... and if you're a non-taster like me, you might discover that fresher -- and weirder -- produce has a more intense flavor that you enjoy, though you might need to acquire a taste for it.
Boil some water, throw the green beans in for just a couple minutes. Remove the beans and put them in a colander to dry. Mince and fry a shallot (or onion) in oil or butter. Throw in the beans, chopped walnuts, and a little bit of feta cheese. Salt to taste (salt is often the difference between bland and tasty).
1.) Spices. there are loads of really interesting, amazing spices out there, but even a simple sautée of olive oil, lemon juice, garlic, salt, and black pepper can make almost any vegetable taste amazing.
2.) Stop eating over-flavoured foods. Doritos etc. have a ridiculous amount of flavouring; if you eat foods like this consistently then you'll desensitise your palette to the subtler but still absolutely lovely tastes of vegetables, in the same way that staring into a spotlight makes it difficult to appreciate the aesthetic qualities of twilight. You can't re-sensitise your palette by simply cutting down on hyperflavoured food; you need to eliminate it almost completely. After a while, you'll discover that brocoli and green beans are wonderful, and Doritos are absolutely brutal on the tongue.