Begin by cooking the shell pasta according to package directions. Then drain the shells, return them to the pot and set it aside. Preheat the oven to 350F. Line a 9x13" baking pan with foil and grease it. Set this aside also.
In a large skillet, heat the olive oil over medium heat. Once it's hot, add the onion and beef and cook, stirring often and breaking up the meat as it cooks, until it is no longer pink. Drain any excess fat out.
Add the garlic to the cooked beef and cook for a minute or so, until fragrant. Then add the crushed tomato, diced tomato, brown sugar and all seasonings, along with the spinach. Once the spinach wilts, remove it from the heat.
Pour the beef mixture into the pot with the drained shells. Add in the ricotta cheese and 1/2 cup of the mozzarella and stir it all together.
Pour the shell mixture into the prepared baking pan. Sprinkle the remaining mozzarella and parmesan evenly over the top and cover with a piece of foil.
Bake for 20 minutes, then remove the top foil and bake uncovered for 10 more minutes, until everything is heated through and the cheese is golden. Serve immediately and refrigerate any leftovers.
