Fish stew made of grooper

Chopped onion braise in home-made olive oil, in a wide shallow pot. Add pealed chopped tomato and braise it for 10 minutes more. Add parsley, garlic and fish in pieces; pour all with home-made white wine (to cover the fish). Boil on light fire for twenty minutes. Shake occasionally, do not stir. At the end add some pepper. Serve with hard-boiled polenta.
Ingredients (for 4 persons): 1 kg of fish, 2,5 dcl of oil, 3 onions, a string of parsley, 1 slice of garlic, 2-3 tomatoes, 2,5 dcl of white wine.