In a bowl, combine yogurt, sour cream, the 1/2 cup onions, and horseradish.
Cover and refrigerate for at least 2 hours or until next day.
In a bowl, combine beef, egg, bread crumbs, the 1/4 cup onions, soy, salt, and hot pepper seasoning.
Shape into 1 1/2 inch-wide patties.
Arrange patties on a lightly greased grill 4 to 6 inches above a solid bed of hot coals.
Cook, turning once, until done to your liking; cut to test
To eat, spear patties with wooden picks; dunk in horseradish sauce