Preparation
Preheat oven to 350 degrees.
In a pan sprayed with nonstick spray, cook onions over high heat until browned (about 3 minutes).
Make sure all excess water has been removed from spinach. In a mixing bowl, combine spinach, onions, cheese wedges, and garlic powder. Mix well.
In a medium casserole dish sprayed with nonstick spray, spread half of the spinach mixture along the bottom.
Layer half of the mushrooms evenly over the spinach mixture in the casserole dish. Then spread the remaining spinach mixture over the mushrooms. Layer the rest of the mushrooms over that.
Sprinkle the top of the casserole with the shredded cheese. Then place dish in the oven to cook for 25 - 30 minutes (until the shredded cheese layer begins to crisp slightly). If you like, season your serving to taste with salt and pepper.