This recipe is from Gourmet magazine.

Steak Sandwiches with Crisp Onions

1 cup thinly sliced onions
2 Tablespoons unsalted butter
1 Tablespoon olive oil
3/4 cup thinly sliced mushrooms
1 garlic clove, minced
3/4 cup beef broth
1 1/2 Tablespoons heavy cream
3 Tablespoons ketchup
1 1/2 Tablespoons Worcestershire sauce, or to taste
1 teaspoon Dijon-style mustard
1 pound grilled steak, cut into 1/8-inch thick slices
1 long loaf of Italian-style bread, halved horizontally and each half buttered lightly, cut into 4 pieces, and toasted

In a skillet cook the onions in the butter and the oil over moderate heat, stirring occasionally, until they are crisp and golden. Transfer them with a slotted spoon to paper towels to drain. In the fat remaining in the skillet cook the mushrooms and garlic over moderately low heat, stirring occasionally, until the mushrooms begin to turn brown. Transfer the mixture to a plate. Add the broth to the skillet and deglaze the skillet, scraping up the brown bits. Add the cream, ketchup ,Worcestershire sauce and mustard. Stir in the mushroom mixture and steak slices. Divide the steak mixture among the bottom bread pieces, sprinkle with some of the onions and top the onions with the remaining bread pieces. Serves 4.