Heat a tbsp. of olive oil in a large pot over medium heat. Once it's hot, add the onion. Cook, stirring frequently, until the onion has softened, about 5-7 minutes. Then stir in the garlic and cook and stir until it's fragrant, just another minute or so. Then add the tomato sauce, sugar and seasonings. Stir and allow it to come to a simmer. Once simmering, reduce the heat to med-low and let it gently simmer while you prepare everything else.
Start a large pot of salted water heating on high heat. Once it reaches a boil, add in the shells. Cook for 1 minute less than the time called for on the box, they will be easier to stuff this way. Once they are done, drain in a colander and immediately rinse them with cold water. This will stop the cooking process, so that they don't get any softer. Set them aside.
In a large skillet, heat the tbsp. of olive oil over medium heat. Add the garlic and cook just until fragrant, just about 30 seconds to 1 minute. Then add in the spinach and stir around slightly. Cook until it has all wilted and reduced, about 3-5 minutes. Remove from heat. drain out any water that may have been released from the spinach, then set aside on a cutting board and allow to cool enough to handle.
In a large mixing bowl, whisk together the ricotta cheese and eggs until blended together. Next whisk in the seasonings, and about 1/4 cup of each of the cheeses. By now, the spinach should be cool enough to handle. Chop it up and stir it into the ricotta mixture.
Preheat the oven to 350F. Grease a 9x13" baking pan and spread a thin layer of the tomato sauce evenly over the bottom of the pan. Then, working one at a time, stuff each shell with 1 heaping tbsp. of the ricotta mixture, until all of the mixture is gone. Place each stuffed shell side by side into the pan, opening side up. Pour the remaining tomato sauce over the top of the shells, then spread over the remaining cheeses.
Cover the baking pan with foil and bake 15 minutes. Then remove the foil and bake an additional 15-20 minutes, until completely heated through. Serve immediately. Refrigerate any leftovers.
