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 saute pan, cook the ground round until browned. Drain meat, reserving 2 tablespoons fat in the pan, and place meat in the same bowl with the spinach. Stir in the cream cheese until blended with the meat and spinach. Saute the chopped onions in the reserved meat fat until transparent. Transfer onions to the bowl with the other ingredients and stir to combine. Add grated cheese, salt, and pepper, to taste, and combine. Let mixture cool and then add the eggs. The filling can be made up to 1 day in advance and refrigerated until ready to use.
Preheat the oven to 350 degrees F.
Cook pasta shells in boiling, salted water until al dente. Drain and cool to the touch. Stuff the shells with the meat mixture. Place shells in a large baking pan and cover with tomato sauce. Cover with foil and bake for approximately 1 hour. Serve.