Roast Beef Sandwich with Melted Cheese and Caramelized Onions

Bahan-bahan (4 porsi)

• 2 teaspoons olive oil
• 1 large white onion (sliced into rings)
• salt and black pepper (to taste)
• 12 oz leftover garlic lovers roast beef (sliced thin)
• 8 oz whole wheat Italian or French bread (cut into 4 pieces)
• 4 oz reduced-fat Swiss Cheese or provolone