Heat 1 tbsp of the oil in a small frying pan and fry the onion until soft. Lightly oil a shallow flameproof casserole with the remaining oil and spread the onion on the bottom.
Cover with the monkfish slices, peas, peppers and mussels.Season with salt and pepper.
In a small saucepan mix together the flour, parsley, saffron, garlic, stock and wine to make a smooth paste. Add a little seasoning then bring to a simmer over a low heat and cook, stirring, until lightly thickened.
Pour over the fish and vegetables and cover with a lid or foil. Bake in a preheated oven 180 C, Gas 6, for about 15 minutes or until the fish has lost its translucent look.
Serve garnished with the lemon slices accompanied by potatoes, salad ( and a full bodied dry white wine or rose if wished. )