For the full effect of this simple but wonderful open-faced sandwich, accept no substitutions! Each ingredient contributes to its distinctive tang.
1 tart apple, such as McIntosh or Cortland, cored and thinly sliced
2 thin slices onion
2 slices sourdough bread
Cheddar cheese, sliced
horseradish mustard
Place sourdough bread on baking sheet. Spread with mustard, then layer with onion and apple. Cover with cheese and place beneath broiler until cheese is melted.