How to Make It
Cook the sausage, stirring often, in a 12-inch ovenproof nonstick skillet over medium-high until cooked through and browned, about 8 minutes. Reduce heat to medium. Add the tomatoes, Italian seasoning, and ½ teaspoon of the salt; cook, stirring occasionally, until the sauce begins to simmer, about 4 minutes. Meanwhile, stir together the ricotta, egg, 1 cup of the mozzarella, and the remaining salt in a medium bowl.
Remove the skillet from heat; set aside 3 cups of the sauce in a bowl. Preheat broiler with oven rack 6 inches from heat. Place a layer of lasagna noodles over the sauce in the skillet, breaking the pieces to fit as necessary. Top with 1 cup of the reserved sauce, ¾ cup of the ricotta mixture, and 3 cups of the spinach. Repeat the layers twice with the remaining noodles, sauce, ricotta mixture, and spinach, omitting the spinach from the top layer.
Return the skillet to heat; cover and cook over medium until the noodles are softened and the sauce begins to bubble, about 20 minutes. Top with the remaining 1 cup of spinach; cover and cook just until the spinach wilts, about 1 minute. Uncover and top with the remaining 1 cup of mozzarella. Broil until the cheese begins to brown, 4 to 5 minutes. Sprinkle with the basil.