Melt butter in a saucepan over medium heat. Add garlic and cook for 1 minute. Then add in flour and continue whisking.
Gradually add milk and whisk to remove any flour lumps. Bring the sauce to a boil and then reduce the heat to low flame.
Add parmesan cheese, herbs, mustard powder, salt, and pepper and continue whisking until the sauce is smooth.
Serve the sauce immediately with pasta, pizza or fried chicken.
If you want to store the sauce for later use, let it cool completely and pour it into an airtight jar. Keep in the refrigerator and use it within 5 days.