Bring 4 – 5 qts of water to a rolling boil, add 2 tbs of salt, gently add pasta, be sure to add extra shells as you will lose a few, cook for approximately 10 minutes, or it tests al dente. Drain and rinse in cold water. If pasta is starchy, you can toss in olive oil to prevent from sticking, but use sparingly as it will also prevent the cheese from sticking.
While pasta is cooking start your sauce:.
In an 6-8 qt pan brown meats over a medium hot burner, work it with a spoon until meat crumbles, add onion, then cook until dark brown, If there is an excessive amount of fat, drain and, optionally, rinse. Add garlic and season to taste. Cook until garlic is fragrant, use caution not to burn. Add sauce & paste, reduce to a simmer. Continue to simmer until ready to serve, adjust seasoning as necessary.
While meat sauce is cooking thaw and rinse and your spinach, I like to use the microwave , but you can rinse it under hot water. After draining all the water you can in a colander wrap the spinach in a very clean towel and twist to get the remaining water out, this is a critical step. When you get done the spinach will be VERY dry. Crumble the spinach in a large bowl, add ricotta, ¾ of the mozzarella, ¾ of the parmesan, mix, add egg, mix add seasonings/garlic and mix. Be sure to add enough salt here, the spinach really comes alive when seasoned properly.
Preheat oven to 350 degrees F.
I like to bake the noodles in 8x12 foil pans, but you can use a single ½ sheet pan or a large casserole dish or a pair of 9x13 pans. Whatever you decide, lightly grease the pans, ladle in a generous amount of sauce. Each shell will take about 2 tablespoons of filling. They are very full, but not too top heavy. I fit about 20 shells in an 8x12 pan. Once the shells are filled and arranged, spoon a small amount of sauce on each one, sprinkle remaining cheeses on the top, cover with greased foil and bake for 25 minutes.
If you are making the shells ahead of time you will need to adjust your cooking time, bake at 325 for 15 minutes or so, then raise temp to 350 and bake for an additional 20-25 minutes. Remove the foil and bake for another 5 minutes, until cheese is melted. At this point you can turn on the broiler (top element) and brown the cheeses.
Remove from oven and let sit for 10 minutes (or so) while you warm up your garlic bread.