Preheat oven to 350 degrees. Cook mushrooms, onions, and garlic until just soft. Remove from heat and allow to cool. Cook spinach in microwave for 1-1/2 minutes; let cool. Mix all ingredients together except reserved mozzarella cheese, pasta shells, and spaghetti sauce in a large bowl with a mixing spoon. Spray a 9-x13-inch baking dish with cooking spray. Use a 1-ounce scoop (or a spoon) to fill shells. Add stuffed shells to baking dish and top with spaghetti sauce. Bake 45 minutes, remove, and top with reserved mozzarella cheese. Bake an additional 10 minutes until cheese is melted.