Spray the bottom and sides of your Crockpot (AKA slow cooker)
Pour oil in a frying pan and add onions and garlic and cook until soft. Add ground beef and continue to cook until beef is no longer pink. Add the pasta sauce and heat together. In a separate bowl, mix the ricotta cheese, milk and egg together until smooth. Add the other cheeses and mix until thoroughly combined.
Put one-fourth of the meat mixture in the bottom of the Crockpot. Add a layer of noodles (may have to break to get them to fit), then one-third of the cheese mixture followed by a layer of noodles, one-fourth of the meat mixture, a layer of noodles, one-third cheese mixture, layer of noodles, one-fourth of the meat mixture, layer of noodles, remaining cheese mixture, remaining noodles, and top with the remaining meat mixture.
Cook on low in Crockpot for 3-4 hours.