Grilled Patty Melt With Balsamic Caramelized Onions | Oregonian Recipes

Ingredients

• 2 tablespoons extra-virgin olive oil
• 1 tablespoon unsalted butter
• 1 teaspoon kosher salt
• 4 tablespoons balsamic vinegar, divided
• 3 large yellow onions, sliced into rings
• 1 pound ground sirloin
• 1/4 teaspoon kosher salt
• 1/4 teaspoon ground black pepper
• 8 slices of fresh rye bread, toasted
• 8 large slices of Swiss cheese, or a combination of Swiss and American