Combine the mayonnaise, ketchup, onion, pickle relish, Worcestershire sauce, lemon juice, paprika, and salt in a medium-sized bowl. Whisk to combine.
Transfer the dressing to an airtight container and refrigerate until ready to serve. For best results, allow the dressing to rest in the refrigerator for at least 12 hours, to allow the flavors to meld, before serving.
Notes
Want to turn your Thousand Island dressing into Russian dressing? Add a dash more plain or sweet paprika and 2 teaspoons of horseradish. Adjust both to taste.