While the pasta is cooking, chop the onions, garlic and parsley. Set them aside.
In a large shallow pot or sauté pan (with high sides), add the sausage. If the sausage is in a casing, make a small cut on one of the ends, squeeze the meat out, and discard the casing.
Stir and using a spatula, break apart the sausage into small pieces as it cooks.
When the sausage starts to brown, add the onions, garlic and olive oil. Stir. Continue to stir while cooking for about 3 minutes or until the onions start to turn translucent.
In a small bowl, add the canned diced tomatoes. With clean hands squeeze the tomatoes so there are no big chunks, and add them to the pan.
Add the tomato sauce to the pan. Fill the empty tomato sauce can with water and add the water to the pan. Stir the sauce.
Add parsley, oregano, red pepper flakes, salt, and pepper. Stir. Turn the heat down to low and let the sauce simmer (uncovered) for about 15 minutes, stirring occasionally.
Add the pasta to the sauce. Gently stir the pasta into the sauce. Top with grated Parmesan cheese and parsley and serve while warm.