This cold sauce fuses mayonnaise with mustard, resulting in a condiment that can be used with many dishes.
Whisk together Dijon mustard,egg yolk, lemon juice, 1/4 teaspoonsalt, and 1/8 teaspoonpepper in a bowl. Pour inoil in a slow, steady stream,whisking constantly untilemulsified and thick. Whiskin grainy mustard, horseradish,and water. (Thin withmore water if too thick.)
Food safety note: The egg in this recipe is not cooked.