Roast them in olive oil, seasoned with salt and pepper. If you like a sweet-and-sour flavor, sprinkle them with just a touch of balsamic vinegar too.
If they are small, wash them well and roast them whole. If they're large, peel them and cut them into hunks like you would if you were roasting potatoes.
I put them in while roasting meat. An hour should be the least amount of time--you want to roast them at least until they're tender. Extra roasting time is a good thing since they'll start to carmelize. If I make them with roast beef, I'll give them 1 1/2 to 2 hours in a 350 oven.