Roast Beef and Fermented Beet "Borscht" Sandwiches

Name: Roast Beef and Fermented Beet "Borscht" Sandwiches
Ingredients: ½ cup, Greek yogurt; ¼ cup, mayonnaise; 2 tablespoons finely, grated fresh horseradish; , Kosher salt and pepper; 13 ounces, roast beef, sliced thin; ¼ cup, beet kvass; 2 cups fermented, beets, cut into ¼-inch strips; 4 soft kaiser, rolls, halved horizontally; 1⅓ cups, fresh dill, chopped into 1-inch sprigs