Create a roux by warming the two tablespoons of butter in a large skillet and adding one tablespoon of flour. Whisk well until all of the flour is incorporated. If you like your sauce on the thicker side, use 2 tablespoons of flour.
Slowly add in 1 cup of milk. Once the flour, butter and milk have combined, add 1/2 cup of Gruyere, 1/4 Cup Cheddar, and 1/4 Cup Parmesan..
At this point, the sauce should be fairly thick. If you wish for it to be thicker, bring the cheese sauce to a slight boil and then remove it from the heat. If you want to thin it out a bit, add more milk. But do it sloooowly.
Now it is ready to enjoy! I used this sauce for a gnocchi mac n cheese. Yum!