Preheat oven to 375 degrees. Over medium heat, sauté onions in butter until soft, lightly browned and caramelized.
Turn heat to medium-high, add Port or Madeira, and cook until liquid has almost evaporated. Set aside until serving.
Split baguette down the center, leaving loaf intact. Place cheese in bottom of baguette. Top with roast beef and caramelized onions. Wrap baguette in aluminum foil and bake for 20-25 minutes until hot.
To serve, remove foil and slice baguette in half. Serve each portion with a small cup of hot au jus or beef broth for dipping.