Poach goose. Cut it into serving portions and sauté pieces until lightly browned as in recipe for Garlic Goose.
Sour Sauce
Wash and clean sorrel carefully; stew in 4 tablespoons each butter and stock until tender. Drain, reserving sorrel liquor. Cook gooseberries in a little water with sugar to taste until tender. Drain. Purée sorrel and gooseberries, and combine in a saucepan with remaining butter, dry white wine and ¼ pint sorrel liquor. Bring to a boil; reduce heat and simmer for 15 to 20 minutes. Season to taste with salt, freshly ground black pepper and sugar. Sauce should be tart.
To serve: place sautéed goose pieces on a heated serving dish. Garnish with sprigs of watercress and serve with Sour Sauce.