Slash loaf into slices (not all the way through). Melt margarine, butter and mustard. Add onion and stir. Cut cheese package in half and halves into triangles. Insert 2 triangles into bread slices. Place bread on foil (large enough to completely cover). Spoon mixture over cheese bread and wrap in foil. Freeze or refrigerate. Refrigerated: 300°, 15 minutes, covered, and 15 minutes, uncovered. Frozen: 400°, same.