Boil onions in a medium sized pot of water until tender, about 10 minutes. Drain and place in a 2 quart baking dish. Preheat oven to 350°F. While onions are boiling, sauté mushrooms in butter over medium heat until soft. Stir in flour and cook for 1 minute. Mix in milk, salt, pepper and sage and continue stirring until smooth and thickened. Pour mushroom sauce over onions. Bake for 20 minutes. Sprinkle with parsley and serve.