Step 1

Melt butter in a large stockpot over medium-high heat.  Add flour, and stir until combined.  Cook for 1 minute, stirring occasionally.  Then pour in 1 cup of the water, and stir (or whisk) until the mixture is completely smooth and begins to thicken.  Gradually pour in the remaining water and milk, stirring until evenly combined.

Step 2

Stir in the pasta, salt, garlic powder, and mustard until combined.  Then continue cooking, stirring occasionally, until the mixture just reaches a simmer.  (Btw, I recommend grating your cheese while the pasta cooks to save time!)  Reduce heat to medium-low to maintain the low simmer.  Continue cooking, stirring occasionally, for about 9-10 minutes, or until the pasta is al dente.  (Be careful not to overcook the pasta!)

Step 3

Remove from heat, and stir in the cheeses until melted.  Taste, and season with additional salt (and black pepper, if you'd like), as needed.

Step 4

Serve immediately, and enjoy!!