1 tablespoon extra-virgin olive oil
1 large white onion, thinly sliced
1 teaspoon granulated sugar
2 boneless, skinless chicken breasts, halved crosswise and pounded to 1-inch thickness
4 slices Havarti cheese
4 hamburger buns
2 cups baby spinach
In a large skillet, heat oil over medium heat. Add onion and sugar; cook 25 minutes or until onions are deeply caramelized, stirring frequently. Heat grill to medium-high. Clean and lightly oil hot grates. Cook chicken until cooked throughout, about 10 minutes, turning once. Place cheese slices on chicken pieces, cover and cook 1 to 2 minutes or until cheese is melted. Build sandwiches, by layering spinach, then chicken, then onions on buns.