Step 1: Prepare the Meat Sauce. In a large skillet or Dutch oven, brown the Italian sausage (casings removed) and ground beef over medium-high heat, breaking it up with a spoon. Drain any excess fat. Add the chopped onion and cook until softened, about 5-7 minutes. Stir in the minced garlic and cook for 1 minute more until fragrant.
Step 2: Simmer the Sauce. Stir in the crushed tomatoes, tomato paste, tomato sauce, water (if needed for desired consistency), sugar, chopped fresh basil, fennel seeds, Italian seasoning, salt, and black pepper. Bring to a simmer, then reduce heat to low, cover, and let it cook for at least 30 minutes (or up to 1-2 hours for deeper flavor), stirring occasionally. Stir in the ¼ cup fresh parsley at the end of cooking.
Step 3: Prepare the Cheese Mixture. In a medium bowl, combine the ricotta cheese, large egg, and 2 tablespoons of chopped fresh parsley. Season with a pinch of salt and pepper and mix well.
Step 4: Preheat Oven and Assemble. Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish. Spread a thin layer of meat sauce on the bottom of the dish. Lay 3-4 lasagna noodles over the sauce (they shouldn't overlap excessively). Spread half of the ricotta cheese mixture evenly over the noodles. Top with a layer of meat sauce, then sprinkle with about a third of the grated mozzarella and a quarter of the Parmesan cheese.
Step 5: Continue Layering. Repeat the noodle, ricotta, sauce, mozzarella, and Parmesan layers. Add another layer of noodles, then the remaining meat sauce, and top generously with the remaining mozzarella and Parmesan cheese.
Step 6: Bake the Lasagna. Cover the baking dish tightly with aluminum foil. Bake for 25 minutes. Remove the foil and bake for an additional 30-35 minutes, or until the top is golden brown and bubbly. If the cheese is browning too quickly, you can loosely tent it with foil.
Step 7: Rest and Serve. Once baked, remove the lasagna from the oven and let it rest for at least 15-20 minutes before slicing and serving. This allows the layers to set, making it easier to cut and serve cleanly.