First, prepare the cheese layer. In a medium bowl, combine the ricotta cheese, 1 cup mozzarella cheese, 1 cup parmesan cheese, Italian seasoning, and salt. Mix well and set aside for later.

Prepare the meat sauce. Heat the olive oil in a large pot or saucepan over medium/high heat.

When olive oil is fragrant, add onions, salt, carrot, and celery to the pan. Saute on medium/high for 5-7 minutes.

Next, add ground beef to the pan and cook for about 4-5 minutes until the beef is almost cooked fully.

Add the marinara sauce and tomato sauce to the pan and mix until all ingredients are combined. Bring to a boil and then immediately remove from the heat.

Assemble the lasagna. Begin by greasing the slow cooker with olive oil or cooking spray.

Using a 1/3 cup scoop, scoop 1/3 cup of the sauce onto the bottom of the slow cooker. Spread the sauce around evenly.

Next, place two lasagna noodles onto the bottom of the slow cooker, lengthwise. Break a separate lasagna noodle into fourths and place 1/4 of a noodle at each end of the long lasagna noodles.*

Spoon 1/4 cup of the cheese mixture over the noodles and spread it out evenly using a spoon or spatula. Finally, spread out 2/3 cup of sauce on top of the cheese.

Repeat this process until all of the sauce, cheese, and noodles are gone. Make sure that the last layer is a layer of sauce and that the noodles are fully coated. If you let the noodles cook without sauce on top, they will end up hard.

Cover the slow cooker and cook on high for 2 hours. With 30 minutes left, add the last half cup of mozzarella cheese to the top of the lasagna.

Uncover the slow cooker and cut the lasagna into even lasagna pieces and enjoy!