Line a 10 inch casserole with spinach which has been seasoned with salt, onion and butter. Sprinkle with 1/2 cup grated cheese.

Arrange sauteed mushrooms over the spinach. Season with garlic salt. Cover with remaining cheese. Bake for 20 minutes at 350 degrees or until cheese is melted and browned.

This casserole may be prepared and refrigerated until it is to be baked.

Makes 6 to 8 servings.