Steak-Lovers' Leftovers (Sandwiches) - cooking recipe

Ingredients

2 tablespoons unsalted butter, plus extra for bread
1 tablespoon olive oil
1 cup thinly-sliced onion
3/4 cup thinly-sliced mushroom
1 clove garlic, minced
3/4 cup beef broth
1 1/2 tablespoons heavy cream
3 tablespoons catsup
1 1/2 tablespoons Worcestershire sauce
1 teaspoon Dijon mustard
1 lb grilled steak or 1 lb cooked roast beef, thinly-sliced
1 loaf long Italian bread

Preparation

Heat 2 tablespoons butter and olive oil in large skillet over medium heat; add onions and cook, stirring occasionally, until golden.
(Note: The original recipe says to cook until\"golden and crispy\". I've never managed to get them crispy, but they're delicious just caramelized.) Using a slotted spoon to allow excess oil to drip back into the skillet, remove onion slices to paper towels to drain.
Reduce heat to moderately low and add mushrooms and garlic to skillet.
Cook in remaining oil, stirring occasionally, until mushrooms begin to turn brown; transfer mixture to a small bowl.
Pour broth into skillet and deglaze, using a wooden spoon to scrape up all the brown bits on the bottom of the pan.
Add cream, catsup, worcestershire and mustard.
(The recipe doesn't say to, but at this point I let it simmer a little while to reduce sauce and thicken a bit.) Cut bread loaf in half horizontally and butter each half lightly.
Cut each half into 4 equal pieces and toast in oven, buttered side up.
Stir sliced beef and mushroom mixture into sauce and heat thoroughly.
Place beef slices on bottom slices of bread, spooning desired amount of sauce over each slice.
Sprinkle caramelized onions over each sandwich and place other half of bread slices on top.