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 in the oil and onion. Season with salt and pepper and cook until the onion is slightly softened, about 2 to 3 minutes. Add in the ground beef and cook, stirring occasionally, until browned and cooked through. That should take about 8 minutes. Add in the lentils, garlic, and Italian seasoning and continue to cook for 1 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 cottage cheese, pesto, egg, and 1 cup of Grana Padano. 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 cheese mixture and stir well to combine.
To assemble the lasagna, spread about a quarter of the sauce across the bottom of the prepared casserole dish then top with a layer of 3 lasagna sheets. Spread over half of the cheese mixture then continue layering with a quarter of the sauce, 3 lasagna sheets, the remaining cheese, another quarter of the sauce, and the remaining 3 lasagna sheets. Top with the remaining sauce, being sure to cover every inch of the noodles.
Scatter over the remaining ½ cup of Grana Padano as well as the grated mozzarella.
Spray the dull side of a piece of aluminum foil with non-stick cooking spray. Cover the lasagna, dull side down, and bake for 45 minutes. Carefully remove the foil and continue to bake for 15 to 20 minutes or until golden brown on top.
Allow the lasagna to sit for at least 10 minutes before slicing and serving.
