Creole Remoulade Sauce

Ingredients

• 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