For the Sauce
In a large pot over medium heat, begin browning the ground beef.
Chop the onion, bell pepper, and zucchini into large chunks and puree them in a food processor.
Move the browned ground beef to the side of the pot and add the pureed veggies to the other side. Season with salt and pepper. Cook everything together for about 15 minutes, stirring occasionally.
Once the beef is fully browned, add the garlic, Italian seasoning, and brown sugar, stirring well.
Stir in the tomato paste and crushed tomatoes, then simmer over low heat while you prepare the rest of the dish.
For the Filling
In a bowl, combine the ricotta cheese, egg, seasonings, and half of the shredded cheese. Mix well.
Prepare the Pasta
Set a large pot of water to boil. Add a pinch of salt and cook the manicotti for about 9 minutes. Drain and rinse briefly under cold water to cool enough to handle.
Assemble the Dish
Spread 1 to 2 cups of sauce on the bottom of a 9x13-inch baking dish.
Carefully fill each manicotti shell with the ricotta mixture using a spatula (or a piping bag for easier filling). After filling, roll the manicotti back up and place it seam side down in the sauce.
Top with the remaining sauce and sprinkle with the remaining shredded cheese.
Bake
Bake the manicotti at 375°F for 30 to 40 minutes, or until the dish is heated through and bubbly.
For a golden, caramelized top, broil the dish for an additional minute or two.