Preheat the oven to 375°F. Spray a 9×13-inch baking dish with non-stick cooking spray.

Heat the olive oil in a large saucepan over medium heat. Add the diced onion, mushrooms, sausage, and ground beef. Cook, breaking up the meat with a wooden spoon or spatula, for 7-8 minutes, or until fully browned. Drain excess fat if needed.

Stir in the marinara sauce, tomato sauce, Italian seasoning, brown sugar, nutmeg, 1 teaspoon salt, and ½ teaspoon black pepper. Reduce heat to medium-low and simmer for 7-8 minutes, until slightly thickened.

In a medium bowl, mix together the ricotta, Parmesan, basil, egg, lemon zest (if using), ½ teaspoon salt, and ¼ teaspoon black pepper. Stir until smooth and well combined.

Assemble the lasagna: Spread ⅓ of the meat sauce in the bottom of the prepared baking dish. Layer with half of the lasagna noodles, breaking them as needed to fit. Spread half of the ricotta mixture evenly over the noodles. Sprinkle with 1 cup of the mozzarella.

Repeat the process: Spread another ⅓ of the meat sauce, then layer with the remaining noodles, the rest of the ricotta mixture, and finish with the last ⅓ of the meat sauce. Top with the remaining 1 cup of mozzarella.

Cover the dish with foil and bake for 35 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is melted and lightly browned. For an extra golden top, broil for 3-4 minutes at the end.

Let the lasagna rest for 10-15 minutes before slicing. Garnish with chopped basil before serving, if desired.