In a medium-size bowl, combine onion, capers, parsley, mayonnaise, cayenne pepper, and horseradish until well blended.
Store, covered, in refrigerator until ready to use.
Makes 1 1/2 cups.
* Be careful when adding the horseradish. Add a small amount at first, taste, and only then do you add more horseradish (if you desire).
Homemade Tartar Sauce Recipe: https://whatscookingamerica.net/tartarsauce.htm