Savory Balsamic Onion Burger with Caramelized Onions - ioanacooks

Ingredients

• 2 tablespoons unsalted butter
• 1 large yellow onion, sliced into 1 cm thick rings
• 60 ml balsamic vinegar
• 1 tablespoon brown sugar
• Pinch of salt
• 450 g lean ground beef (10% fat)
• 4 slices provolone cheese
• 4 whole grain burger buns
• Salt and freshly ground black pepper, to taste
• Fresh arugula leaves, for serving
• Mayonnaise, for serving