Bring a pot of salted water to a boil. Cook the pasta in salted water, using a timer. Cook 1 mins before the time on the package.
While the pasta is cooking, heat up some olive oil in a nonstick skillet over medium heat. Fry the sausage in little balls or break up into pieces with a wooden spoon.
When the sausages are browned and crisp, add garlic and chili flakes, if using. Fry until crispy and fragrant.
Add 2 cups of pasta water to the pan and swirl. Once reduced by about half, remove from the heat until the pasta timer goes off, then add the almost al dente pasta to the skillet and turn the heat back up to high.
Swirl the pasta until al dente and a thick sauce has formed and coated each strand or piece of pasta. Add the cheese, then stir lightly to melt. Serve immediately, topped with more parmesan.