Brown beef and onion in a skillet 8–10 minutes, breaking up meat, until beef reaches 160°F. Drain fat; stir in marinara and water. Mix ricotta, 1 cup mozzarella, Parmesan and Italian seasoning in a bowl.

Oil a 6-quart oval slow cooker. Spread one quarter of meat sauce on bottom. Add one third of noodles, broken to fit a single layer with minimal overlap, half the ricotta mixture and another quarter sauce.

Repeat with one third noodles, remaining ricotta and one quarter sauce. Finish with remaining noodles and sauce, covering all exposed pasta.

Cover; cook on low 3–4 hours, until noodles are tender when pierced and center reaches 165°F. Add remaining mozzarella during the last 15 minutes. Turn off; rest uncovered 20 minutes before scooping into 6 portions.