Preheat the oven to 400 degrees. Spray an 8x8 baking dish with non-stick spray.
Heat a medium skillet over medium high heat and add the sausage. Crumble and cook for 5 minutes or until browned. Add in the onion and cook until softened, about 3 minutes more.
Add the spinach, spaghetti sauce, tomato sauce and parsley and cook for 2 more minutes. Taste, and add salt and pepper as needed. Remove from the heat.
In a medium bowl, whisk together the ricotta cheese, egg, Parmesan, Italian seasoning, parsley, salt and pepper.
Spread about ¼ - ½ cup of the meat sauce on the bottom of the prepared dish. Layer lasagna noodles on top, breaking the noodles to fit if necessary.
Spread half of the ricotta cheese mixture on top of the noodles followed by 1 cup of the mozzarella cheese. Spread ⅓ of the meat sauce mixture on top of that.
Add another layer of noodles, followed by the remainder of the ricotta cheese mixture. Then add another cup of mozzarella, followed by another ⅓ of the meat sauce mixture.
Add the final layer of noodles, then the last of the meat sauce mixture. Sprinkle the remaining 1 cup of shredded mozzarella cheese on top.
Cover with foil and bake for 35 minutes, then remove the foil and bake for an additional 10 minutes or until the cheese is golden brown and bubbly.