Clean the onion heads and cut them into slices about 3/4″ (2 cm). Arrange them one next to another in a tray that you've smeared with 1-2 tbsp olive oil.
Mix the rest of the olive oil with the balsamic vinegar and pour the resulting mixture over the onions. Season with salt and black pepper. Bake about 40-45 min. in a preheated 392°F (200 °C) oven until browned and softened.
Before serving, sprinkle with finely chopped fresh parsley.