Place yolk, mustard, lemon juice, water, cayenne, garlic, and bread in bowl of food processor. With processor running, slowly drizzle in vegetable oil until smooth, mayonnaise-like consistency is reached. Transfer sauce to medium bowl. Whisk in olive oil in steady stream until smooth, then season to taste with salt and pepper. Rewhisk before serving with grilled fish.

Makes 1 cup, enough for four pieces of fish