•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, if necessary
•Nonstick cooking spray
•4 slices Swiss cheese
•4 hamburger buns
•2 cups baby spinach
1.In large skillet, heat oil over medium heat. Add onion and sugar; cook 25 minutes or until onions are deeply caramelized, stirring frequently. This step can be done up to 3 days in advance. Re-warm onions in microwave or in a small pan on grill before proceeding.
2.Preheat grill for direct grilling over medium-high heat. Spray chicken with nonstick spray. Transfer chicken to hot grill rack; cover and cook 10 minutes or until chicken loses its pink color throughout and internal temperature reaches 160 degrees F, turning once.
3.Place cheese slices on chicken pieces. Cover and cook 1 to 2 minutes or until cheese is melted and golden brown. Build sandwiches using hamburger buns, spinach, chicken and caramelized onions.