Combine sugar, vinegar, orange juice, onion flakes, dried mustard, black pepper, and salt in a blender or food processor. Mix or process 1 minute.
1¼ cups granulated sugar; ultrafine sugar can be used, ¾ cup white vinegar, 2 tablespoons orange juice, 2 tablespoons dried onion flakes, 2 teaspoons dried mustard, ½ teaspoon freshly ground black pepper, ¼ teaspoon kosher salt