Add the butter and oil to a large fry pan. When hot add in the onion. Saute until translucent. (About 3-4 minutes). Add in the garlic. Saute for 1 minute. Remove from the stove and set the onion and garlic aside. Add the beef and Italian sausage to the pan. Saute both until browned. Drain off the grease. Add the onion and garlic back into the pan. Add in the tomato paste, diced and crushed tomatoes, herbs, and salt and pepper. Stir until well combined. Simmer on low heat while you make up the cheese sauce.
Combine the ricotta, parmesan, and eggs in a large mixing bowl. Whisk until smooth. Add in the parsley.
Grease a 6-quart slow cooker with butter or non-stick spray. TIP: You can also use a slow cooker liner for easy cleanup!
Begin assembling the lasagna. First, add in 1/3 of the meat sauce. Then top the meat sauce with uncooked noodles. (About 3-4) (You may have to break to fit in the slow cooker.) Top noodles with 1/3 of the cheese sauce. Sprinkle with mozzarella cheese. Repeat layers two more times, ending with mozzarella cheese. Sprinkle with more parsley and fresh basil.
Turn the slow cooker to low and cook lasagna for 4 hours. ***For crispier top remove the slow cooker's cover 30 minutes before done. Place a piece of paper towel over the top and continue cooking. Allow the lasagna to sit for 15 minutes before cutting and serving.