Horseradish Mustard Mayonnaise
You would typically find this sauce served alongside a thick cut of prime rib. The blend of mellow mayonnaise, pungent horseradish, and sharp Dijon mustard complements steak so well that it is no surprise that it does the same for a beef burger. A rich salmon burger would also take beautifully to the sauce. For a touch of extra flavor and color, try adding a few tablespoons of chopped fresh herbs such as chives, dill, or tarragon.