French Onion Chicken Sandwiches 1 tablespoon extra virgin olive oil 1 large white onion, thinly sliced 1 teaspoon granulated sugar Olive oil nonstick cooking spray 2 boneless, skinless chicken breasts, halved crosswise and pounded to 1-inch thickness, if necessary 4 slices Swiss cheese 4 hamburger buns 2 cups baby spinach