Cook bacon in a saucepan until crisp, remove bacon but leave the grease.
Add butter to the pan once it is melted, add the onion and cook until transparent (3-5 minutes).
Add the flour to the butter/onion mixture and stir until it becomes paste-like and a light golden brown.
Whisk in the mustard and beer, stirring constantly.
Slowly add in milk and stir constantly.
Simmer over medium heat until the mixture is smooth and thick (5-10 minutes).
Mix in the cheddar and parmesan cheese and stir until completely blended.
Stir in bacon bits and pasta.
Video
Notes
To avoid lumpy sauce, be sure to continuously stir the roux when preparing it.
When adding the cheese, remove from heat first then mix in the cheese. The cheese can separate and make for a grainy and unpleasant sauce if it gets too hot or boils.