Wash the fish, dab dry, sprinkle with lemon juice and let it steep a little. Clean, wash and drain the peppers and cut them into pieces. Peel onion and cut into rings. Heat 20 g fat in a frying pan and brown the onion rings slightly in it. Take them out and stew the peppers for about 2 minutes in the pan. Mix with the onions, season with salt and pepper and place in an ovenproof dish. Season the fish fillet with salt and place it on the vegetables.
Melt the remaining fat in a pot and sauté the flour in it. Add stock and milk while stirring, bring to the boil and simmer for about 5 minutes, stirring occasionally. Stir in 30 g cheese and season with salt, pepper and paprika. Pour over the fish and vegetables. Sprinkle with remaining cheese and bake in the preheated oven (electric cooker: 200 °C / gas: level 3) for 40-45 minutes. Rice tastes good with it