Pre-heat 30cm/25cm size serving dish in oven at gas mark 4.
Boil potatoes and mash with half of the butter and half of the creme fraiche. Lightly brown onions and capers for 2 minutes. Add fish, peppers and mushrooms to pan and fry for 2 more minutes. Add stock and vermouth and poach for 10 minutes, then pour off reduced stock.
Make roux by stirring flour into melted butter and adding milk and stock from poaching pan. Melt sliced cheese into sauce and stir in the remaining creme fraiche.
Lay fish on base of heated serving dish and cover with sauce and vegetables. Spread mashed potato over the mixture and decorate with tomatoes.
Cook through in oven for 45 minutes and sprinkle with parsley to serve. Serve with braised courgettes, peas or broccoli.
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