Ingredients

• 3 medium onions, thinly sliced (3 cups)
• 1/2 stick (1/4 cup) unsalted butter
• 10 oz mushrooms, thinly sliced (4 cups)
• 1 1/2 lb sliced rare roast beef, cut into 1-inch pieces (5 cups) and pieces separated
• 1/4 cup water
• 4 (6-inch-long) sections soft Italian bread
• 1/2 lb sliced provolone or mild Cheddar

Instructions

1. Preheat broiler.
2. Cook onions in butter in a 12-inch heavy skillet over moderately high heat, stirring occasionally, until pale golden, about 5 to 7 minutes. Add mushrooms and cook, stirring occasionally, until mushrooms are golden, 5 to 7 minutes. Stir in beef, water, and salt and pepper to taste and cook, stirring, until heated through, about 1 minute.
3. Halve bread horizontally without cutting all the way through and open cut sides, then put bread in a shallow baking pan. Divide beef mixture evenly among bread and top with cheese. Broil 6 inches from heat until cheese is melted, 1 to 2 minutes.