1. In a bowl, mix sugar and flour. Add lemon peel, juice, and 2 tablespoons butter; mix well. In a small bowl, beat yolks to blend with milk and stir into lemon mixture.
2. In a deep bowl with a mixer on high speed, beat egg whites and cream of tartar until they hold stiff, moist peaks. Gently fold egg whites into lemon mixture.
3. Butter a 4- to 5-cup straight-sided shallow baking dish or soufflé dish and set dish in a slightly larger baking pan. Pour lemon mixture into dish and set pan with dish on oven rack. Fill outer pan with boiling water to the depth of 1 inch.
4. Bake in a 350° oven until pudding top springs back when lightly touched, 35 to 40 minutes. Lift dish from water.
5. Serve dessert warm, scooping down to bottom of the dish to include pudding that forms beneath cake. Serve with berries.