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.

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 350°F. 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.