Brown Italian sausage and ground beef in a skillet over medium-high heat until no pink remains. Drain well.
Stir spaghetti sauce into the browned meat and warm through.
In a bowl, combine cottage cheese, egg, Parmesan, and Italian seasoning until uniform.
Spray a 6-quart slow cooker. Layer one-third meat sauce, 3 broken noodles, half the cottage cheese mixture, and 1 cup mozzarella.
Repeat with one-third meat sauce, 3 noodles, remaining cottage cheese mixture, remaining 3 noodles, and final one-third meat sauce.
Cover and cook on low for 4 hours, until noodles are tender.
Sprinkle remaining 1 cup mozzarella on top during the last 30 minutes.
Turn off slow cooker and let lasagna rest 10 to 15 minutes before serving.
Notes
Use regular dry lasagna noodles, not no-boil noodles, for the best slow cooker texture. Do not reduce the sauce because the dry noodles need that liquid. Let the lasagna rest before serving so the layers firm up. Avoid lifting the lid during cooking because it adds time.