Remove shell from your hardboiled eggs. Remove yolks and mince egg whites into little pieces.
In a bowl mix together mustard, mayo, horseradish and egg yolks until you have a thick paste. Add in minced egg whites. Mix together. Add salt and pepper as desired.
Spread about half of the egg mixture on on slice of bread. Top with a thin layer of sprouts. Top with second slice of bread. Slice in half and serve.