1Melt butter in a large pot over medium-high heat. Add flour and stir until combined, cooking for 1 minute, stirring occasionally.
2Gradually pour in the water and milk, stirring until evenly combined.
3Stir in the pasta, salt, garlic powder, and mustard until combined and continue cooking until the mixture just reaches a simmer. 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. Remove from heat, and stir in the cheeses until melted. Season with additional salt and black pepper as needed.