Preheat oven to 350°F. Arrange tuna in a baking pan. Pour fish stock over tuna and season with pepper and nutmeg. Cover with a buttered piece of foil. Bake 15 minutes, until fish flakes when tested with a fork, lifting the foil and basting fish occasionally. Transfer tuna to a warm serving dish. Pour pan liquid into a heavy nonreactive saucepan and boil 4-5 minutes, or until mixture is reduced in volume by half. Stir in tomato sauce and oregano and simmer until hot. Add lemon juice and serve sauce over fish. Sprinkle with parsley.
