Melt the butter in a medium sauté pan over medium-low heat. Whisk in the flour, stirring constantly to prevent lumps, until the mixture darkens a little, about 1 to 2 minutes.Turn up heat to medium. Pour in the milk and continue whisking to break up any lumps. When the milk thickens to the consistency of thin gravy, 2 to 3 minutes, turn off the heat and stir in the cheeses and salt, until well incorporated.Serve warm over vegetables, pasta, rice, or whatever you want!