3 T butter
2 T finely chopped onions
2 T flour
1 cup milk
1/3 cup horseradish mustard

Melt butter and sauté onion until tender. Remove from heat, stir in flour; then milk to make a smooth sauce. Add mustard and heat until warm. Serve warm with beef fondue or roast beef. Delicious cold on ham sandwiches.

Variation: For a real horseradish sauce, use 1/3 cup grated horseradish instead of the mustard.