Carefully and gently pick over 1 pound crab meat, discarding any bits of shell. Transfer crab meat to a medium mixing bowl.
For the dressing, in a separate mixing bowl whisk together ½ cup mayonnaise, 1 tablespoon ketchup, 1 tablespoon lemon juice, 1 teaspoon lemon zest, ½ teaspoon Worcestershire, ½ teaspoon Tabasco, ½ teaspoon Old Bay seasoning, ½ teaspoon kosher salt, and a few turns of cracked black pepper.
Add dressing to crab meat and gently fold together with a rubber spatula. Fold in chopped celery and scallions. Season to taste with additional salt, pepper and lemon.
Cover and refrigerate for at least 30 minutes to allow flavors to meld (or up to 4 hours).
To serve as a dip, pile crab salad into a serving bowl. Sprinkle with additional Old Bay seasoning. Using a microplane, grate fresh horseradish generously over the top of the crab. Garnish with celery leaves. Serve with celery stalks and buttery crackers.
Or, to serve as a salad, spoon crab salad onto Bibb lettuce leaves and garnish as above. You can also serve open-faced on toasted bread or stuffed into toasted hoagie rolls.