Preheat oven 200 degrees C. Wash fish and clean out stomach thoroughly. Cut 4 diagonal slits on each side of the fish. Place a few thin slices of red pepper in each slit.
Mix condensed tomato soup and 200ml of cider together in a saucepan. Heat until it starts to boil, then reduce heat and simmer until it has reduced by half.
Meanwhile, to make the stuffing: melt butter and cook sliced onion and rest of red peppers until they have softened. Add breadcrumbs, lemon juice and 2 tablespoons of reduced sauce. Season with salt and pepper to taste.
Stuff each fish with stuffing and place in a deep baking dish. Gently pour over sauce followed by remaining cider.
Cover with foil and cook for 25 minutes. Remove foil and bake for a further 10 minutes.