Created by breezermom on August 12, 2016
Step 1: Clean mushrooms with a damp paper towel. Remove and finely chop stems. Set mushroom caps and chopped stems aside.
Step 2: Cook the spinach according to the package directions; drain well, and press between paper towels to remove the excess moisture.
Step 3: Dip mushroom caps in melted butter; place in a 13 x 9 x 2 inch baking dish, cap sides down.
Step 4: Saute reserved chopped mushroom stems and onion in any remaining melted butter in a skillet. Add spinach, stir well. Set aside.
Step 5: Combine the egg yolk, 2 1/2 tablespoons Parmesan cheese, salt, nutmeg, and pepper in a small bowl; stir well. Add to the spinach mixture, stirring well.
Step 6: Spoon spinach mixture evenly into mushroom caps, sprinkle evenly with the remaining 1 1/2 tablespoon Parmesan cheese. Bake at 325° for 20 to 25 minutes or until the stuffed mushrooms are thoroughly heated.