In a medium bowl, combine mayonnaise, yogurt, onions, capers, lemon juice, tarragon, mustard, horseradish, salt, and pepper; mix until well blended.
Cover and store in refrigerator until ready to use.
Makes approximately 1 1/2 cups.
* Learn how easy it is to make Homemade Mayonnaise.
Tarragon Mayonnaise Recipe: https://whatscookingamerica.net/sauces_condiments/tarragonmayonnaise.htm