Preheat oven to 350 degrees.
Add olive oil and onion to a skillet over medium heat. Saute for 5 minutes. Add in the sausage and garlic and continue cooking until browned and cooked through. Spoon out any excess fat and discard. Season with salt and pepper.
Add 2 cups of the marinara sauce to the cooked sausage mixture. Stir well.
Pour the remaining marinara into the base of a greased 9 x 13 casserole dish and spread it evenly on the bottom.
Add half the ravioli over the marinara. Then add half the sausage mixture covered by half the cheeses. Repeat this process and use the remaining cheeses to top.
Cover with aluminum foil and bake for 30 minutes. Take the foil off and cook for an additional 10-15 minutes until cheese is bubbly and browned. Garnish with fresh basil. Enjoy!