In a bowl, combine the meat, carrot, celery, mushrooms, and garlic.
Season with salt and pepper. Set aside.
In the bottom of the slow cooker, spread 1/2 cup of tomato sauce. Cover with a layer of lasagna noodles. Break them if needed to fit.
Spread one-third of the meat mixture over the noodles. Cover with 1 cup of tomato sauce and sprinkle with 1/3 cup of Parmesan. Cover with another layer of lasagna noodles. Cover with the ricotta cheese.
Continue with a layer of lasagna noodles. Add another third of the meat mixture. Cover with another layer of noodles, the remaining meat mixture,
cup of tomato sauce, and the remaining 2/3 cup of Parmesan. Top with the remaining tomato sauce and sprinkle with the mozzarella cheese.
Cover and cook on low for 4 hours. Cooking time may vary depending on the type of slow cooker you have. If the pasta is tender when a knife is inserted, the lasagna is ready. Try not to leave the lasagna on warm after it is cooked because the pasta will get soggy. Serves 6-8.