In a large skillet, cook the sausage over medium-low heat, breaking it up as it cooks. Add the onion and garlic, and cook until the onion is translucent. Stir in the tomato sauce and basil, bring to a simmer and remove from stove.
In a medium mixing bowl, blend together the ricotta, egg, ½ cup mozzarella and the Parmesan cheese.
Cover the bottom of a 2-quart slow cooker with ½ cup tomato sauce mixture. Breaking them up to fit, cover the sauce with 2 pieces of lasagna. Spoon 1/3 of the cheese mixture over the lasagna and cover with 1/4 of the remaining sauce. Break and add 2 more pieces lasagna, spoon on 1/2 of the remaining cheese and top with 1/3 the remaining sauce. Add 2 more pieces lasagna and cover with the last of the cheese and half the remaining sauce. Cover with the last 2 pieces of lasagna and the last of the sauce.
Cover, set to low-heat and cook for 6 hours. Sprinkle the remaining ½ cup mozzarella over the lasagna, turn off the cooker and let it sit for 15 minutes before serving.