In a skillet cook the onions in the butter and the oil over moderate heat, stirring occasionally, until they are crisp and golden and transfer them with a slotted spoon to paper towels to drain. In the fat remaining in the skillet cook the mushrooms and the garlic over moderately low heat, stirring occasionally, until the mushrooms begin to turn brown and transfer the mixture to a plate. Add the broth to the skillet and deglaze the skillet, scraping up the brown bits. Add the cream, the ketchup, the Worcestershire sauce, and the mustard and stir in the mushroom mixture and the steak slices. Divide the steak mixture among the bottom bread pieces, sprinkle it with some of the onions, and top the onions with the remaining bread pieces. Serves 4. Gourmet February 1990