Brown the sausage in a skillet over medium heat, breaking it up as it cooks. Add the diced onion and minced garlic and cook until the onion softens.
Stir the pumpkin puree, marinara sauce, Italian seasoning, salt, and pepper into the sausage mixture until well combined.
Spread a thin layer of the sauce mixture across the bottom of the slow cooker. Break 3 noodles to fit and lay them over the sauce.
Dollop ricotta over the noodles and sprinkle with some mozzarella and Parmesan. Repeat the layers twice more, ending with a final layer of sauce and the remaining mozzarella and Parmesan on top.
Cover and cook on low until the noodles are tender and the cheese has melted into the layers.