Heat your oven to 375ºF and lightly grease a 9- by 13-inch casserole dish with non-stick cooking spray.
Place a large skillet over medium-high heat and add oil and onion. Season with salt and pepper and cook until the onion is slightly softened about two to three minutes. Add the sausage meat and ground beef, stirring occasionally, until browned and cooked. That should take about eight minutes. Add in the garlic, cherry tomatoes, and Italian seasoning and continue to cook for a minute. Stir in the tomato paste, followed by the crushed tomatoes, remove from the heat, and set aside.
In a large bowl, stir together the ricotta cheese, pesto, egg, and one cup of Parmigiano Reggiano—season with a pinch of nutmeg and some salt and pepper. Place the spinach in a few pieces of paper towel and squeeze out any excess moisture. Add to the ricotta mixture and stir well to combine.
Spread about a quarter of the sauce across the bottom of the prepared casserole dish to assemble the lasagna, then top with three lasagna sheets. Spread over half of the ricotta mixture, then continue layering with a quarter of the sauce, three lasagna sheets, the remaining ricotta, another quarter of the sauce, and the remaining three lasagna sheets. Top with the remaining sauce, and cover every inch of the noodles.
Scatter over the remaining half a cup of Parmigiano and the grated mozzarella or fontina.
Spray the dull side of a piece of aluminum foil with non-stick cooking spray. Cover the lasagna, flat side down, and bake for 45 minutes. Carefully remove the foil and bake for 15 to 20 minutes or until golden brown.
Allow the lasagna to sit for at least 10 minutes before slicing and serving. Enjoy!