In a large skillet sauté onions and sausage in oil until meat is browned. Stir spaghetti sauce into the mixture and simmer for 15 minutes. Add spinach and season with salt and pepper. Remove from heat.
Meanwhile cook ravioli according to the package directions. Spread a little of the meat sauce on the bottom of a casserole dish and then cover with half the ravioli. Spoon half the meat sauce on top, then half of the cheese.
Repeat layering in same order, ending with the cheese on top. Bake at 350⁰ F in preheated oven for 15 minutes. Let stand 10 minutes before serving. To serve, sprinkle with parsley. Enjoy!