Peel and halve or quarter the onions. Peel and finely chop garlic. Put the onions and garlic in an ovenproof dish and add olive oil and balsamic vinegar to taste, salt and season with black pepper from the mill.
Stew in the preheated oven at 180 degrees until the onions are almost cooked.
Cut the feta into cubes and add to the onions. Braise for another 10 minutes.
Then remove from the oven and sprinkle with chopped basil.