Make the dressing: Whisk the mayonnaise, ketchup, horseradish, Worcestershire sauce and salt to taste in a small bowl until smooth.
Cook the bacon in a skillet over medium heat until crisp, then transfer with a slotted spoon to a paper-towel-lined plate to drain.
Roughly chop the lettuce, then toss with some of the dressing in a serving bowl. Top with the tomatoes and scallions. Scatter the bacon and blue cheese over the salad; season with pepper and sprinkle with the parsley. Serve with more dressing on the side.