Place fillets of sole in a well-buttered flameproof gratin dish or shallow casserole. Blend vermouth, melted butter and tomato concentrate, and pour over fish. Season to taste with salt and freshly ground black pepper. Cook over a high heat until fish flakes easily with a fork.
Add cream and simmer gently for a minute or two, shaking the pan continuously so that the sauce will thicken gradually.
To serve: place fish fillets on a heated serving dish; pour over sauce; sprinkle with finely chopped truffle or parsley, and garnish with several crescents of flaky pastry. Serve immediately.