Arrange the prepared potatoes in the bottom of a large shallow ovenproof dish. Pour on half the stock.
Brush each fillet with lemon juice and sprinkle with 1 tsp old bay. Dot with knobs of butter.
Cover and cook for approximately 35-45 minutes or until the fish is cooked and potatoes are just tender.
Without disturbing the fish and potatoes, drain off and reserve the stock. Cover the dish and keep warm.
For the sauce, melt the remaining butter in a saucepan and fry the onion until golden. Stir in the cornstarch, tomato paste, cayenne pepper,thyme and remaining old bay. Make the drained stock up to 2 1/2 cups using remaining stock and blend into the pan.
Add the mushrooms and peppers. Bring to the boil, stirring and simmer for 5 minutes.
Pour the sauce over the fish and garnish with twists of lemon. Serve with green vegetables.