Heat a saucepan on medium heat and drop in the butter.
Once the butter has melted and started to bubble add in the flour all at once and mix to form a paste, then allow to cook for about a minute or two once combined, making sure to keep stirring.
Stirring all the time add the milk a little at a time until it has all been incorporated and you have a smooth white sauce.
Add the cheese and allow to bubble.
Remove from the heat and stir in the pepper/onion/garlic mix.
Combine with the pasta.