In a large skillet, over medium high heat, saute the chopped onion until translucent. Add in the minced garlic and stir until fragrant.
Add in the ground beef, salt, pepper, and oregano. Brown the meat until no longer pink.
Pour in the jar of sauce. Add approximately ½ cup of water to the jar, cover and shake, then pour into the skillet. Let simmer until warmed through.
While the sauce is cooking, to a large bowl, add the ricotta cheese, egg, 1 cup of shredded mozzarella cheese, ¼ cup of grated parmesan cheese, and salt & pepper to taste. Stir until fully combined.
To the bottom of a 6 quart slow cooker, add approximately 1 ½ cups of the meat sauce. Top with uncooked noodles. Break to fit into a single layer (mine took 3 noodles). Top with spoonfuls of the ricotta mixture then spread evenly. Repeat layers (sauce, noodles, cheese) ending with sauce.
Top with remaining shredded mozzarella and parmesan cheese.
Cover and cook on LOW for 3.5 hours or until noodles are soft. Uncover and let set for approximately 10 minutes before slicing.
Depending on the size of your slow cooker, you will likely only use half of the box of lasagna noodles - mine used 9 noodles.
Each slow cooker cooks differently. Mine was done in 3.5 hours on LOW. Yours may take 4-5 hours.