- 1 large egg yolk
- 1/2 cup chopped green onions (green and white parts)
- 1/2 cup chopped yellow onions
- 1/4 cup fresh lemon juice
- 1/4 cup chopped celery
- 3 tablespoons Creole or other whole-grain mustard
- 3 tablespoons ketchup
- 3 tablespoons chopped fresh flat-leaf parsley
- 2 tablespoons chopped garlic
- 2 tablespoons prepared horseradish
- 1 teaspoon salt
- 1/4 teaspoon cayenne
- 1/4 teaspoon freshly ground black pepper
- 3/4 cup vegetable oil
Combine all the ingredients except the vegetable oil in a blender or a food processor fitted with a steel blade and process until combined, about 30 seconds. With the motor running, slowly pour in the oil and continue to process until fairly smooth, about 30 seconds.
Transfer to an airtight container and refrigerate for at least 1 hour before using. The sauce will keep for up to one week in the refrigerator.