Thaw and drain the spinach, pre-cook the sausage (optional), and combine the ricotta, Parmesan, and egg as directed. Spread a thin layer of the pasta sauce in the bottom of the slow cooker, and top it with 3 noodles cut to fit and laid side by side. Spread 1/3 of the ricotta cheese mixture over the noodles. Top it with 1/3 of the spinach, (and all of the sausage, if you are using it), and then 1/3 of the remaining sauce. Repeat the layers two more times with the noodles, ricotta cheese mixture, spinach, and sauce. Cook on low 4 - 6 hours or until noodles are tender. Turn off the slow cooker, then top with the mozzarella cheese and let it sit, covered, 10 minutes before serving.
(Slow cooker cooking times may vary—Get to know your slow cooker and, if necessary, adjust cooking times accordingly.)