Bring a large pot of salted water to a boil. Add the spaghettini and cook according to package directions until al dente. Reserve 1 cup of pasta water before draining.
While the pasta is cooking, heat the olive oil in a large skillet over medium heat. Add the garlic and red pepper flakes, and cook for 1-2 minutes, or until fragrant. Be careful not to burn the garlic. Add a pinch of salt and pepper.
Add the drained pasta to the skillet with the garlic and olive oil. Toss to coat and turn heat off.
Gradually add the reserved pasta water, 1/4 cup at a time, mixing in until the pasta is coated in a creamy sauce. You may not need the entire cup.
Stir in ½ cup of the cheese. To serve, add another pinch of pepper, parsley, and top with the remaining cheese.