1.Preheat Oven To 400 Degrees F (200 Degrees C). 2.Spray Rounded Ends Of Mushroom Caps With Vegetable Spray And Place Oiled Side Down On Baking Sheet. 3.Place Spinach In Microwave-Safe Bowl, Sprinkle With A Few Drops Of Water, And Microwave On High For One Minute. Chop Cooked Spinach; Mix With Cheddar Cheese, Cottage Cheese, Garlic Powder, And Salt. Spread Spinach Mixture Onto Prepared Mushroom Caps. 4.Bake In Preheated Oven Until The Mushrooms Are Tender, About 12 Minutes. Serve On Hamburger Buns.