In a frying pan over medium heat, cook the Italian sausage and chopped onion together until onions are tender and sausage is broken up into small pieces, cooked through with no pink left, about 5-7 minutes.
Heat and spray the crock pot with cooking spray while you are cooking the sausage and onion and preparing the other ingredients. That way it's warmed up and ready to go!
Transfer the cooked Italian sausage and onion to a greased slow cooker that holds at least 6 quarts. Add marinara sauce, diced tomatoes, and dry noodles and mix well.
Make ricotta filling by combining the ricotta cheese, 1 cup mozzarella cheese, parmesan cheese, egg, Italian seasoning, garlic powder, salt, and pepper in a bowl. Mix well.
Stir ricotta mixture gently into noodles and meat sauce. Make sure all the noodles are covered. Then sprinkle the remaining 2 cups mozzarella cheese on top.
Cook on high for 2.5 hours or low 4-5 hours until noodles are tender. Enjoy!Optional: sprinkle with fresh chopped basil for a pop of flavor!
If the slow cooker cooks longer than 2.5 hours on high heat, the noodles may get a little soggy and the edges may get a little crispy, just like in the oven. We love the crispies!