Bring a large pot of water to boil and cook the shells according to the lesser time on the directions. Drain and rinse with cold water. If you overcook them, they'll fall apart as you're stuffing them. Believe me. ;)
Meanwhile, thaw the spinach. Heat the oil in a skillet and cook the onion until soft, a few minutes, then add the garlic and cook another minute or two. Squeeze the spinach to remove the excess moisture and add to the pan and stir to mix up, then turn the heat off and let it cool.
In a small bowl, mix the Ricotta, 1 cup of the Mozzarella, and the Parmesan along with the eggs. When the spinach mix has cooled off a bit, stir it into the cheeses.
Preheat the oven to 350.
Pour half the sauce into an 8x8 pan. Use a large spoon to fill each shell with the cheese and spinach mixture. Lay them in the pan as you go. If you have extras, you can freeze for another day, as I did. Pour the rest of the sauce over the shells when you're done, then top with the leftover Mozzarella. Bake uncovered for half an hour or so.