In 10-inch skillet, melt 3 tablespoons of the butter over medium-high heat. Cook mushrooms and garlic in butter until mushroom liquid is absorbed. Transfer mixture to large bowl. In skillet, melt remaining 1 tablespoon butter over medium-low heat; add spinach. Cover; cook 5 minutes or until wilted. Add spinach to mushroom mixture. Stir in ricotta cheese, Parmesan cheese, egg, 1/2 teaspoon of the salt and the pepper. Spoon mixture evenly into pasta shells. Place in baking dish.