Put the onion, mustard, lemon juice, parsley, egg, salt and cayenne in a food processor or blender and blend for 30 seconds. With the motor running, slowly drizzle in the vegetable oil, the mixture will thicken slightly. Check seasoning. Yield: approximately 1 1/2 cups

*RAW EGG WARNING

Food Network Kitchens suggest caution in consuming raw and lightly-cooked eggs due to the slight risk of Salmonella or other food-borne illness. To reduce this risk, we recommend you use only fresh, properly-refrigerated, clean, grade A or AA eggs with intact shells, and avoid contact between the yolks or whites and the shell.