In a small bowl, combine the mayonnaise, ketchup, sweet pickle relish, chopped onion, white vinegar, sugar, salt, and black pepper.
Stir well until all ingredients are thoroughly combined.
Taste and adjust seasoning, adding more salt or pepper if desired.
Cover and refrigerate the dressing for at least 30 minutes to allow the flavors to meld together.
Serve chilled as a salad dressing or dipping sauce for various dishes.
Notes / Tips / Wine Advice:
Feel free to adjust the ingredient quantities to suit your taste preferences.This homemade Thousand Island dressing is versatile and pairs well with salads, sandwiches, burgers, and as a dipping sauce for appetizers like the crispy Reuben egg rolls.