In a medium sauce pan, melt the butter over medium low heat. Once melted, add the garlic and cook about 1 minute. Then add the flour and whisk constantly and cook about 1 more minute.
Slowly pour in the milk, whisking constantly. Once all the liquid is added, increase heat just a touch and whisk occasionally while allowing to cook until gently simmering and bubbling around the edges of the pan.
Once bubbling and thickened, turn off the heat and add the herbs, salt, pepper, and both kinds of cheese. Whisk to combine.