Preheat the oven to 350 degrees F (175 degrees C). Melt butter in a 9 x 13-inch baking dish while the oven preheats. In a large bowl, whisk together the eggs, milk, flour, salt, pepper, baking powder and garlic until well blended. Add the mushrooms, spinach and cheese, and stir to blend evenly. Tip the baking dish to coat with melted butter, then pour the spinach mixture into the pan. Bake for 30 minutes in the preheated oven, until firm and golden. Cut into bars, and serve warm. Yield: 12 Servings.