Lightly boil the shells in a large pot of salted water so they open easily - about 15 minutes. Drain and reserrvefor later.
Heat a saute pan to medium add the oil and saute the onions until the are soft - about 10 minutes. Add the chopped spinach and wilt for a couple of minutes. Remove from the heat.
In a large mixing bowl combine the ricotta, mozzarella, parmesan egg, lemon juice, rosemary, nutmeg, ground black pepper and spinach mixture then mix well. Leave a bit of the parmesan and mizzarella cheese out of the filling to top the shells witth.
Take about ½ cup of the tomato sauce and spread it out across the bottom of a pie plate or similar sized baking dish.
Using a large soup spoon scoop a heaping spoonful of the filling and put it inside a shell. Place the shell in the dish and continue filling shells and placing them in the dish until they are all filled.
Drizzle the remaining sauce over the shells and top with the leftover cheese.
Cover the dish with aluminum foil and bake in the oven for 30 minutes.
Notes
1. For the best pasta, the water should be salty like the sea. I always season it with at least a tablespoon of sea salt.2. Shells are known to break while cooking. I always throw a few extra in the pot in case that happens.