In a large pot of boiling water, cook the pasta according to the package directions, or until tender but slightly firm to bite (not mushy).
While the pasta cooks, combine the cheese, milk, flour, mustard (if you’re using it), salt and pepper in a blender or food processor, and pulse until well blended.
When the pasta is ready, drain it well and return it to the pot. Add the cheese mixture and stir over medium heat for a few minutes, until it bubbles and turns in to cheese sauce. (Letting it bubble will ensure it reaches its maximum thickening potential, and will get rid of any floury texture.) Serve immediately.