Melt butter in a saucepan. Add flour and cook for 1-2 minutes.
Add milk a small amount at a time, whisking smooth after each addition.
Add onion powder and white pepper.
Continue to cook over medium heat, whisking continually until slightly thick.
Remove from heat, and add cheeses. Stir until melted.
(Nutrition information provided is an estimate and will vary based on cooking methods and brands of ingredients used.)