Mushrooms Florentine

4 tablespoons butter, melted, divided
1 pound medium-size whole mushrooms
1 teaspoon salt
1/4 cup finely chopped onion
2 (10 ounce) packages frozen chopped
    spinach, thawed and drained
1 cup shredded Cheddar cheese, divided
Garlic salt, to taste

Preheat oven to 350 degrees F.

Place 2 tablespoons of the butter in a large skillet over medium heat. Add the mushrooms and cook until browned, browning the cap sides first. Stir the salt, onion and remaining 2 tablespoons butter into the spinach. Line a 10 x 7-inch casserole with the spinach mixture. Sprinkle 1/2 cup of the cheese over the spinach. Arrange the browned mushrooms over the cheese; season with the garlic salt and top with the remaining 1/2 cup cheese. Bake for 20 minutes or until the cheese is melted and browned. Serve hot.