Add the olive oil to a large skillet and heat it over medium-high heat. Add the onion and sausage to the skillet. Crumble and cook the sausage until it's browned and cooked through. Add in the garlic and cook for another 30 seconds, until fragrant.
Turn the heat down to medium and pour in the marinara sauce, balsamic vinegar, and Italian seasoning. Stir everything together.
Stir in the ravioli until they're coated in the sauce. Top with the cheese and then cover with foil and place the skillet on the middle rack of the oven. Bake for 30 minutes, until it's hot and bubbly.
Remove the foil and turn the oven to broil. Broil for several minutes or until the cheese starts to get brown and crispy. Garnish with chopped parsley if desired then serve.
Notes
The leftover ravioli will keep for up to 4 days in an airtight container in the fridge. Reheat the leftovers in the microwave or covered with foil in the oven.