- 3 tablespoons prepared horseradish
- 1/4 cup yellow mustard seeds, coarsely crushed
- 1 tablespoon vegetable oil
- 4 kaiser rolls, halved horizontally
- 3 cups (about 6 ounces) thinly sliced iceberg lettuce
Whisk mayonnaise, sour cream, and horseradish in medium bowl to blend. Season with salt and pepper. Form ground beef into four 1/3-pound burgers, each about 3 3/4 inches in diameter and 1/2 inch thick. Sprinkle both sides of burgers with salt and pepper. Spread mustard seeds on small plate. Press both sides of burgers firmly into mustard seeds.
Heat vegetable oil in heavy large skillet over medium-high heat. Add burgers and cook until brown on both sides and pink in center, about 4 minutes per side. Arrange roll bottoms on each of 4 plates. Transfer burgers to roll bottoms. Stir lettuce into horseradish mixture to coat. Divide lettuce mixture atop hamburgers. Cover each burger with roll top and serve.