Roast Beef and Cheddar Panwiches with Caramelized Onions and Horseradish Mayo

2 tablespoons extra virgin olive oil

2 tablespoons butter

3 medium onions, thinly sliced

2 teaspoons worcestershire sauce

1/2 cup mayonnaise

3 tablespoons horseradish

3/4 cup melted butter

1 teaspoon dried mustard

1 tablespoon Worcestershire sauce

12 slices sour dough bread

3/4 pound deli sliced roast beef

1/2 pound (approx.) sliced cheddar cheese (I used 17 slices)

  1. Preheat oven to 350 degrees.
  2. In a large skillet, add olive oil and 2 tablespoons butter over medium high heat.  When butter is melted, add onions and cook until golden brown and caramelized, about 15 minutes, stirring often.  Stir in 2 teaspoons Worcestershire sauce.  Remove from heat.
  3. Stir together the mayonnaise and horseradish.  Set aside.  
  4. Melt 3/4 cup butter in a small saucepan; whisk in the dried mustard and 1 tablespoon Worcestershire sauce.  Brush the bottom and sides of a 9 x 13 inch metal pan or glass casserole dish generously with about a 1/3 of the butter mixture.  
  5. Spread one side of each slice of bread with the mayonnaise.  Place 6 slices of bread, mayo side up, in the bottom of the pan evenly.  
  6. Place the beef evenly on the bread, followed by the cheese. (It’s okay to overlap them and cover all the bread.)  Spread the caramelized onions on top of the cheese.  Place the remaining 6 slices of bread mayo side down on the onions, lining up with the other bread.  
  7. Generously brush the top of the bread with the remaining butter mixture.  Bake for 20 minutes, or until cheese is melted and the top is golden brown, placing under broiler if necessary to brown the top.  Let cool for 10 minutes before cutting apart into individual sandwiches.