In a medium bowl, whisk together the mayonnaise, ketchup, relish, olives, and onion. Season to taste with salt and pepper.
In a large bowl, pour 1 cup of the dressing over the lettuce and toss to combine.
Divide the dressed lettuce between 4 chilled salad plates, then top each with ¼ of the crabmeat.
Drizzle the crab with the remaining dressing.
Serve cold, with lemon wedges and Tabasco sauce, if desired.
