1. Into a casserole pour the oil and layer up the onion, potatoes and tomatoes.
2. Arrange the fish fillets on top.
3. Sprinkle the lemon juice, wine and capers (if using) and season to taste.
4. Cover casserole with a lid and braise the fish for 20 minutes until the fish is firm and flakey.
5. Serve the straight from the casserole along with the vegetables.
6. Accompany with crusty bread.
You can use any other white fish steaks as well but increase cooking time by 5 minutes.