Cook sausage over medium heat for approximately 8 minutes, stirring occasionally until sausage is no longer pink. Drain.
Stir in tomato sauce, garlic, and onions. Let the sauce simmer for a few minutes while you prepare your cheese filling.
Next, add 1 cup of mozzarella, ricotta, Boursin, and Parmesan cheese to a mixing bowl and fold until well blended. Refrigerate the remaining mozzarella cheese while the lasagna slow cooks.
Drizzle the bottom of slow cooker with olive oil and spread evenly on the bottom of the slow cooker. You can also spray the bottom with cooking spray.
Spoon sauce onto the bottom of slow cooker to cover the bottom, then top with 5 lasagna sheets. Note: It's approximately ¾ cup of sauce for each sauce layer in my slow cooker.
Next, spread with half of the cheese mixture.
Top the cheese mixture with another layer of sauce to cover, then top with 5 more lasagna sheets.
Now you will add your remaining cheese mixture, then top with another layer of sauce to cover.
Top sauce layer with remaining 5 lasagna sheets and the remainder of sauce.
Cover and cook on low heat setting for 4 to 6 hours or until noodles are soft and tender.
About 20 minutes before done (or serving), sprinkle the top of lasagna with the remaining mozzarella cheese.
Cover and let stand for about 10 minutes or until cheese is completely melted.
Top with fresh basil for garnish.
Serve with garlic bread and ENJOY!