Brown the ground beef in a skillet over medium heat until no longer pink, then drain excess fat.

In the same skillet, stir in the marinara sauce, tomato paste, and water until well combined.

In a separate bowl, whisk the egg and mix with the ricotta cheese, parmesan, oregano, salt, and pepper.

Spread 1 cup of the prepared meat sauce across the bottom of a 6-quart slow cooker.

Break lasagna noodles to fit and create a single layer over the sauce.

Spread one-third of the ricotta mixture over the noodles and top with one-quarter of the mozzarella cheese.

Repeat layers of meat sauce, noodles, and ricotta mixture two additional times.

Spread the remaining meat sauce over the final layer of noodles and top with the remaining mozzarella cheese.

Cover and cook on low for 4 to 5 hours or on high for 2 to 3 hours until noodles are tender.

Turn off the slow cooker and let the lasagna rest for 15 minutes before slicing and serving.