Spray the inside of a 4-quart slow cooker with the cooking spray. Heat the oil in a large skillet over medium-high heat. Add the mushrooms; cook 3 minutes, stirring occasionally. Add the spinach; cook 3 minutes more or until the mushrooms are tender and spinach is wilted, stirring occasionally. Stir in the tomatoes with their juices, tomato sauce, Italian seasoning, and salt. Bring to a boil. Reduce the heat and simmer 3 minutes, stirring occasionally. Meanwhile, combine the ricotta cheese, ¼ cup of the Parmigiano-Reggiano and pepper in small bowl; set aside.
Spread ¾ cup of the sauce mixture over the bottom of the slow cooker. Layer 3 lasagna noodles over the sauce mixture, breaking the noodles to fit. Top with ¾ cup of the sauce mixture, the ricotta cheese mixture and 1 cup of the Italian cheese. Top with ¾ cup sauce mixture and the remaining 3 lasagna noodles, breaking to fit. Spoon the remaining sauce mixture over the noodles.
Cover; cook on low for 5 hours or until the noodles are tender. Sprinkle with the remaining ½ cup Italian cheese and remaining ¼ cup Parmigiano-Reggiano. Cover; let stand 5 minutes or until the cheese melts.