Open-Faced Cheeseburgers with Mushrooms and Onions

INGREDIENTS

• 2 tablespoons olive oil
• 10 ounces white mushrooms, trimmed and thinly sliced
• Coarse salt and ground pepper
• 1 medium red onion, halved and thinly sliced
• 1 teaspoon red-wine vinegar
• 1 pound ground beef chuck
• 1 plum tomato, cored and cut lengthwise into 4 slices
• 2 ounces fontina cheese, cut into 4 slices
• 4 slices sourdough bread (1 inch thick), toasted