Clean the chicken livers. Finely chop an onion head and the garlic cloves. Put them in preheated oil to get braised. Once slightly softened, add the livers and sauté until they change color slightly.
Mix the canned tomatoes with salt, black pepper and chopped parsley. Add 1 tablespoon lemon juice. Transfer the livers to an oven dish, pour in the tomato sauce, top with sliced onion rings and cover the oven dish with foil.
Bake in a preheated moderate oven for 30 minutes. Then remove the foil and bake for another 10 minutes.