Step 1: Cook manicotti shells in a large pot of salted boiling water according to package directions, but undercook by 2-3 minutes until al dente. Drain, rinse with cold water, and lay flat on a clean towel to prevent sticking.
Step 2: In a large skillet or Dutch oven, brown the ground beef (or Italian sausage) over medium-high heat, breaking it up with a spoon. Drain excess fat. Add diced onion and minced garlic; cook until softened, about 3-5 minutes.
Step 3: Stir in the crushed tomatoes, tomato paste, Italian seasoning, sugar (if using), salt, and pepper. Bring to a simmer, then reduce heat, cover, and cook for at least 30 minutes, stirring occasionally.
Step 4: While the sauce simmers, prepare the ricotta filling. In a large bowl, combine drained ricotta cheese, 1 cup of Parmesan cheese, the egg, chopped parsley, salt, and pepper. Mix well until fully combined.
Step 5: Preheat oven to 375°F (190°C). Spread a thin layer of meat sauce on the bottom of a 9×13 inch baking dish.
Step 6: Carefully fill each manicotti shell with the ricotta mixture using a small spoon, pastry bag, or a ziplock bag with the corner snipped off. Arrange the stuffed shells in a single layer in the prepared baking dish.
Step 7: Pour the remaining meat sauce evenly over the manicotti, ensuring all shells are mostly covered. Sprinkle with the remaining 1/2 cup of Parmesan cheese and 2 cups of shredded mozzarella cheese.
Step 8: Cover the baking dish tightly with aluminum foil. Bake for 30 minutes. Remove the foil and bake for an additional 15-20 minutes, or until the cheese is bubbly and golden brown. Let rest for 5-10 minutes before serving.