Barefoot Contessa | Smashed Hamburgers with Caramelized Onions |…

Ingredients

• Canola or grapeseed oil
• 4 cups (¼-inch) sliced red onions (2 medium)
• 1 teaspoon sugar
• 1 tablespoon good red wine vinegar
• 1½ teaspoons dry mustard powder, such as Colman’s
• Kosher salt and freshly ground black pepper
• 1¼ pounds ground beef with 20% fat
• 1¼ cups grated Gruyère cheese (4 ounces)
• 4 sandwich potato rolls, such as Martin’s