1 portobello mushroom cap, cut into 1/2-inch slices
1 tablespoon red wine vinegar
2 slices pumpernickel rye bread
1 tablespoon Dijon mustard
1 tablespoon sliced pimento-stuffed green olives
2 slices Havarti cheese
Preheat oven to 400 degrees F (200 degrees C).
Place mushroom slices in a bowl. Sprinkle mushrooms with red wine vinegar. Lay the bread slices on a baking sheet. Spread mustard evenly on one slice. Arrange the mushroom slices on the slice of bread with the mustard. Place the green olive slices on the other slice of bread. Cover each piece of bread with the Havarti cheese.
Bake in preheated oven until the cheese melts, 5 to 7 minutes. Remove from oven and put the slices together to make a sandwich.