Add the Italian sausage to a large skillet and cook over medium until fully browned.
While the sausage is cooking, dice the onion. Add the onion to the skillet with the cooked sausage and continue to cook and stir over medium heat until the onion is soft and translucent.
Add the crushed tomatoes, tomato paste, and Italian seasoning to the skillet. Stir to combine and bring the sauce up to a simmer.
Once simmering, place a lid on the skillet, turn the heat down to medium-low, and allow the sauce to simmer, stirring occasionally, while you cook the ravioli.
Bring a large pot of water to a boil. Once boiling, add the ravioli and continue to boil for 3-4 minutes, or until cooked through (check the cooking instructions on the package for the recommended cook time). Drain the pasta in a colander.
Preheat the oven to 350ºF. Add 1 cup of meat sauce to the bottom of a casserole dish and spread it to cover the surface.
Layer in half of the ravioli followed by half of the shredded mozzarella and 1 cup of sauce.
Layer in the second half of the ravioli followed by 1 cup of sauce and the second half of the shredded mozzarella.
Cover the casserole with foil and bake in the preheated oven for 20 minutes.
Remove the foil, turn the oven from bake to broil, then return the casserole to the oven. Broil for a few more minutes, watching very closely, or until the cheese is browned. Top with chopped parsley if desired, then enjoy!
*The Italian sausage makes this dish super filling, and it also brings a lot of built-in herbs and spices that give the sauce and casserole flavor. You can substitute non-pork Italian sausage if preferred, but if you substitute plain ground pork or ground beef, you'll want to double up on the seasonings in your sauce.**If you don't have any Italian seasoning, you can substitute a mix of dried basil, dried oregano, rosemary, marjoram, and thyme.***Frozen ravioli makes this dish come together so quickly and easily. Use cheese ravioli for the most lasagna-like experience. If you use beef ravioli, you may want to layer in some cheese with your pasta and sauce before baking and reduce the amount of Italian sausage in the sauce.****You can use plain mozzarella or substitute with a mix of Italian cheeses if you prefer.