Preheat oven to 400 degrees F. In a big pot boil 6 cups of water and cook shells al dente style (according to package), add salt and pepper to taste.
While the shells are boiling Heat a large pan or skillet. Add the ground beef (no oil needed) and brown for 2-3 minutes. Add the chopped onions, garlic, basil, oregano, tomato paste and salt + pepper to taste. Simmer for 2 minutes then add the spinach and simmer for 2 more minutes.
After the shells are cooked (be careful not to overcook because they will start to break apart) and the meat sauce is ready put the mixture in the shells like a tablespoon and half into each one line them up in across a 13x19 casserole dish. Pour the marinara sauce over the top, sprinkle with mozzarella cheese and Parmesan cheese and cook for 15 minutes at 400°F.