In a large skillet over medium heat, brown the ground beef, breaking it up as it cooks. When just about done, add the onion and garlic to the pan and continue cooking until the beef is cooked through.
Add the tomato sauce, tomato paste, salt, and oregano. Stir until well combined. Remove from heat.
In a bowl, mix together the ricotta cheese, egg, Italian seasoning, and Parmesan cheese. Stir well to combine.
Spoon a layer of the meat mixture into the bottom of a slow cooker.
Scatter 1/3 of the broken noodles evenly into the slow cooker.
Top the noodles with 1/3 of the ricotta mixture.
Repeat the layering to make 2 more layers, ending with just sauce and mozzarella cheese on top.
Cover and cook on LOW until lasagna noodles are tender, about 4 to 6 hours.