2 tablespoons unsalted butter
2 tablespoons all-purpose flour
1/4 teaspoon mustard powder
3/4 cup beer
1/2 cup half-and-half
6 ounces grated sharp yellow cheddar cheese
1 tablespoon horseradish
1 1/2 pounds ground beef, preferably chuck
Kosher salt and freshly ground pepper
Vegetable oil, for the grill
4 poppy seed rolls, split
Tomato slices, for topping
2 tablespoons chopped fresh chives