Heat olive oil in a large Dutch oven or pot over medium-high heat. Add ground beef and Italian sausage. Cook, breaking up the meat with a spoon, until browned, about 8-10 minutes. Drain any excess fat.

Add chopped onion to the pot and cook until softened, about 5-7 minutes. Stir in minced garlic and cook for 1 minute until fragrant.

Stir in crushed tomatoes, tomato sauce, tomato paste, beef broth, dried oregano, dried basil, red pepper flakes (if using), bay leaf, and granulated sugar. Season with 1 teaspoon salt and 1/2 teaspoon black pepper. Bring to a simmer, then reduce heat to low, cover, and let simmer for at least 30 minutes, or up to 1 hour, stirring occasionally. The longer it simmers, the more flavorful it will be. Remove the bay leaf before layering.

While the sauce simmers, prepare the ricotta filling. In a large mixing bowl, combine ricotta cheese, grated Parmesan cheese, egg, chopped fresh parsley, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Mix until well combined.

Cook lasagna noodles: Bring a large pot of salted water to a rolling boil. Add lasagna noodles and cook according to package directions until al dente (firm to the bite). Do not overcook. Drain immediately and rinse with cold water to prevent sticking. Lay noodles flat on parchment paper or a clean kitchen towel to cool slightly.

Preheat oven to 375°F (190°C).

Spread about 1.5 cups of meat sauce on the bottom of a 9x13 inch baking dish.

Arrange 3-4 cooked lasagna noodles over the sauce, slightly overlapping if necessary to cover the base.

Spread half of the ricotta cheese mixture evenly over the noodles.

Sprinkle about 1.5 cups of shredded mozzarella cheese over the ricotta layer.

Top with about 1.5 cups of meat sauce.

Repeat the noodle, ricotta, mozzarella, and meat sauce layers once more: arrange 3-4 noodles, spread remaining ricotta, sprinkle with 1.5 cups mozzarella, then top with 1.5 cups meat sauce.

For the final layer, arrange 3-4 noodles, top with the remaining meat sauce, and sprinkle generously with the remaining 1 cup of mozzarella cheese.

Cover the baking dish tightly with aluminum foil (you can spray the underside of the foil with non-stick cooking spray to prevent cheese from sticking).

Bake for 30 minutes.

Remove the foil and bake for an additional 15-20 minutes, or until the cheese is bubbly and lightly golden brown.

Remove 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 preventing it from falling apart.