Put the onions (unskinned) on an ovenproof plate and cover loosely with aluminum foil.
The covering is only to prevent the onion juice from spurting all over the oven, but you must have some air around the onions.
Bake in a hot oven for about 1 hour.
When I am cooking a roast of beef I always bake potatoes and onions in the same oven.
They are all cooked together and so little trouble.