1. Fill large pot with water, add 1 teaspoon of salt and bring to a boil.
2. Meanwhile heat 2-3 tablespoons of olive oil, add chopped onion and sauté for 3-4 minutes, and garlic and sauté 2 minutes more. Add fresh spinach and cook, stirring occasionally, until the leaves begin to wilt, about 5 minutes. Drain the liquids and let cool. Chop the spinach mixture and transfer to a large bowl.
3. To the bowl add ricotta cheese, grated parmesan cheese, salt, pepper, nutmeg and mix well. Transfer to a piping bag.
4. When the water is boiling add the pasta and cook according to package directions. Drain and place in a bowl with cold water.
5. Preheat oven to 360F (180C).
6. Pour 1/2 cup of the tomato sauce into the bottom of baking dish. Stuff each pasta shell with the spinach and ricotta mixture. In a large baking dish, arrange the shells in a single layer, with the open side facing upwards. Add to each stuffed shell one baby mozzarella ball. Pour the remaining tomato sauce on top. Sprinkle with cheese. Cover with foil.
7. Bake for 20 minutes, remove the foil and bake 10-15 minutes more or until the top is golden.