In a small saucepan, melt the butter over medium-low heat.
Add the flour to the melted butter, and cook for 2-3 minutes, stirring, until the mixture is heated and starting to brown.
Slowly add the milk to the saucepan, stirring constantly, until the milk is combined with the flour mixture.
Add the cheeses, ¼ of a cup at a time, whisking between additions, until all the cheese is added and melted.
Stir in the garlic and pepper.
Serve hot over steamed broccoli or cauliflower.