Blanch the lettuce for 3-4 minutes in lightly salted water. Drain the excess water, and put the lettuce into a dish, with the shallot, and pour over 150ml of the wine. Season, dot with the butter, and sprinkle with the sugar. Cover with foil and cook at 150c for approx 45mins. Remove the foil and put the dish back in the oven for 10-15 minutes.
Reduce the remaining wine and fish stock by half then add the double cream, bring back to the boil, season and add a little lemon juice to taste. Turn off heat and add peas to warm through. Season the fish fillets and place in pan skinned side up, cook for 2/3 minutes at a high heat then turn the fillets and put the pan back in the oven for a further 2/3 minutes at 180c.
To serve, cut the stalk end off the lettuce and put it on a plate. Pour on the sauce and the peas. Lay the fillet on top of the peas and sauce, drizzle with a little of the mint oil and serve.