Place frozen spinach in a strainer and run under hot water until thawed.
Squeeze all of the water out and place in a large bowl.
In a large sauté pan, cook the beef, pork, onions and garlic until browned.
Place meat in the same bowl as spinach.
Stir in the cream cheese until blended with the meat and spinach.
Add grated cheese, salt and pepper to taste and stir into meat and spinach mixture.
Let mixture cool and then add egg.
Cook pasta shells until al dente.
Drain and cool to the touch.
Stuff the shells with meat mixture.
Place shells in a large baking pan and cover with tomato sauce.
Cover with foil and bake at 350 degrees for about 1 hour.
Sprinkle with mozzarella cheese and let melt under foil for few minutes after remove from oven.