12 lg. mushrooms
10 oz. pkg. frozen spinach
3 tbsp. butter
1 minced onion
1 egg yolk
1/2 tsp. salt
1/8 tsp. pepper
1/8 tsp. nutmeg
1/4 c. grated Parmesan

Preheat oven to 350 degrees. Clean mushrooms. Remove stems. Chop stems. Cook frozen spinach according to package directions. Drain excess water. Chop spinach. Place mushroom caps into 8 x 12 inch baking dish. Melt butter, add onion and mushroom stems and saute until onion is translucent. Stir in spinach and remove from heat. Combine egg yolk, salt, pepper, nutmeg, 2 1/2 tablespoons of Parmesan cheese and add to spinach mixture. Stuff each individual mushroom cap with mix and sprinkle with remaining cheese. Bake for 30 minutes.