butter lettuce mandarin salad

1/2 cup sour cream
1/4 cup mayonnaise
1/4 cup milk
3/4 teaspoon minced garlic
1/2 teaspoon Worcestershire sauce
1 pinch white sugar
1 pinch ground white pepper
1 pinch ground cayenne pepper
2 heads butter lettuce - rinsed, and torn
1 small sweet onion, thinly sliced
1 (11 ounce) can mandarin oranges, drained
5 ounces fresh sliced mushrooms
salt and pepper to taste