Bake the unpricked pastry shell in a 450 degree F oven for 4 to 5 minutes or just until it begins to puff. Remove from oven. Reduce oven temperature to 375 degree F.

Meanwhile, melt butter in a medium skillet over medium-high heat. Add chicken and onion; cook until onion is tender. Sprinkle cheese over bottom of prebaked pastry shell. Spoon chicken mixture over cheese.

Whisk together eggs, half-and-half, mustard, salt, and cayenne pepper in a medium bowl. Pour over the chicken mixture.

Bake in the 375 degree F oven about 30 minutes or until a knife inserted near center comes out clean. If necessary to prevent overbrowning, cover edge of crust with foil. Let quiche cool for 10 minutes before serving. Garnish with fresh herb sprig, if desired. Makes 6 to 8 servings.