Preheat the oven to 400℉ (200℃) lightly grease a deep 9x13 baking dish with olive oil and set aside.
Heat the olive oil in a large skillet over medium-high heat for 30-60 seconds. Add the sausage and Italian seasoning and cook, breaking the meat into small bite-until fully cooked through. Once browned, turn off the heat, drain off any excess grease, and then stir in the spaghetti sauce and mix until well combined. If you're skipping the meat, simply use the prepared sauce as-is for layering in the casserole.
Spread a thin layer of the prepared sauce on the bottom of the casserole dish. Arrange half of the ravioli in a single layer over the sauce, overlapping slightly if needed. Spoon half of the remaining sauce over the ravioli and spread to cover. Sprinkle half of the mozzarella cheese evenly on top. Repeat the layers with the remaining ravioli, sauce, and mozzarella, then finish by sprinkling Parmesan cheese over the top of the casserole.
Cover the baking dish with foil and bake for 20 minutes. Uncover and bake for an additional 10 minutes if using fresh ravioli and 20 minutes if using frozen ravioli.
Let rest for 10-15 minutes before serving with additional parmesan cheese if desired.
Video
Notes
Ravioli: Use either fresh or frozen cheese or spinach ravioli. There is no need to defrost or boil the ravioli before baking. Note On Sausage: Swap out the italian sausage for ground beef or ground turkey. Or omit the meat altogether. Spice it Up: Add up to ½ teaspoon of crushed red pepper flakes to the marinara sauce.Storage: Store leftovers in an airtight container in the refrigerator for up to 3 days. Reheat individual servings by heating for 30-45 seconds in the microwave on a heat-safe plate covered with a damp paper towel. Make-Ahead Instructions: Assemble and cover tightly. Refrigerate for up to 2 days. Let sit at room temperature for 30 minutes, then bake as directed. To freeze in advance, assemble in a freezer- and oven-safe dish (disposable works well) and cover tightly with two layers of foil. Freeze for up to 3 months. Thaw overnight in the refrigerator, let sit at room temperature for 30 minutes, then bake, adding about 10 minutes to the baking time.