In a small saucepan over medium heat, heat butter and add onions and garlic.
Saute until the onion begins to caramelize and are lightly browning.
Add the mushrooms and cook for about 5 minutes.
Add port and worcestershire and stir together for about 2 minutes.
Add the beef consomme, thyme, peppercorns and bay leaf and bring to boil, reduce heat and simmer for at least 15 minutes.
Strain through a fine sieve and serve in a large ramekin for dipping.
Cut the baguette into quarters and lightly spread aioli or horseradish mayo on one side of the bun (don't want the flavour to overpower).
Add a thick layer of sliced roast beef on both sides of the baguette and cover with sliced provolone cheese and place under the broiler to melt the cheese.
Combine the halves together to make a sandwich and serve with the au jus in a cup or large ramekin and enjoy the dipping!