Peel onions and trim slightly at the top and bottom. Boil the onions gently in salted water until tender (about 30 minutes).
Preheat oven to 350 degrees F (175 degrees C). Butter one shallow baking dish.
Saute the sliced mushrooms in the butter or margarine. Stir in the flour. Stir in the half and half or milk, salt, sage, 1/2 of the lemon peel and all of the lemon juice. Cook, stirring over medium heat until sauce thickens.
Place the cooked onions in the prepared baking dish and pour the mushroom sauce over them.
Bake at 350 degrees F (175 degrees C) for about 20 minutes. Sprinkle the top with the chopped parsley, remaining lemon peel and paprika to taste.
