Complex sentence: Mayfair salad dressing is an anchovy-based salad dressing created at the Mayfair Hotel in downtown St. Louis, location of the first five-star restaurant in Missouri, "The Mayfair Room", which featured an Elizabethan inspired decor.
Split into simpler sentences: Mayfair salad dressing is a salad dressing incorporating anchovies, created at the Mayfair Hotel in downtown St. Louis. It was first served in the hotel's restaurant, The Mayfair Room, the first five-star restaurant in Missouri, which featured Elizabethan-inspired decor.