In a large sauce pan, melt the butter. While whisking, sprinkle the flour into the pan. Cook for 1-2 minutes, stirring out any clumps. Slowly stream in 1 cup of milk and cook until thickened.
Pour in the remaining milk and water, and the Dijon mustard and salt. Mix until evenly combined.
Add the pasta to the pot and bring to a simmer. Adjust the heat so it remains at a simmer until pasta is al dente, about 8-10 minutes.
Remove from heat and let cool for 3-5 minutes. Working in batches, stir in shredded cheese and mix until melted.
