Heat a medium skillet over medium-high heat. Cook the Italian sausage together with the diced onion until the sausage is fully browned and the onion is soft.

Mix in the tomato pasta sauce and salt, stirring well.

Lightly coat the inside of a 2.5-4 quart oval-shaped slow cooker with non-stick spray to prevent sticking.

Spoon a small amount of the sauce mixture into the bottom of the slow cooker.

Break three lasagna noodles to fit, and place them in the slow cooker as the base layer. Spread 1/3 of the sauce mixture over the noodles, then sprinkle 1/3 of the shredded mozzarella and 1/3 of the Parmesan on top.

Repeat these layers two more times: noodles, sauce, mozzarella, and Parmesan.

Cover the slow cooker with the lid and cook on low for 2.5-3 hours, or until the noodles are tender.

Turn off the slow cooker and let the lasagna sit for 10 minutes before slicing and serving. Enjoy!