8. Prepare Sauce
In the now-empty saucepan, melt 3 more tablespoons of the butter over medium heat. When the butter is foamy, whisk in the flour. Continue to cook, whisking constantly, until the flour is golden and nutty, about 3 minutes.
While whisking constantly, gradually pour in the milk. Bring to a simmer, and then continue to simmer, still whisking, until thickened. Remove from the heat and whisk in the sour cream, Worcestershire, Dijon, salt, and pepper.