Cook 2 pkgs. (10-oz. Size) frozen chopped spinach according to package directions. Drain well and squeeze out excess liquid.
Spoon spinach evenly into bottom of shallow casserole (9 x 13 works well). Sprinkle with ½ tsp. Salt, ¼ cup very finely chopped onion, 2 tbsp. Melted butter, andd ½ cup grated cheddar cheese.
Rinse and dry 1 lb. Fresh mushrooms. Cut in halves and sssaute in 4 tbsp. Butter until just crisp-tender. Spoon mushrooms over cheese layer in casserole dish. Sprinkle lightly with ¼ tsp. Garlic powder and top with ½ cup grated cheddar cheese.
Bake in preheated 350 degree oven for 20 to 25 minutes or until bubbly.
Makes 6 servings.