1. In a large saute pan heat the olive oil. Add the onion and cook until softened, about 5 minutes. Then add the garlic and cook for one additional minute. Add the sausage and cook until the meat is browned and fully cooked through.
2. Next, add the marinara sauce and salt, lower the heat to the lowest setting. Cover with a lid and stir ocassionally, until sauce has thickened. Turn off heat. Place the sauce in the fridge until you are ready to heat in eat it if making this ahead of time.
3. Prepare penne according to package directions. Drain it and reserve 1 c. of pasta water. Add the penne to the sauce in the pan and swirl to incorporate. If sauce seems too thick thin it out with a few splashes of pasta water. Scoop into a pasta bowl, top with grated cheese and serve.