French Onion Chicken

Name: French Onion Chicken
Ingredients: 5 tablespoons unsalted butter , divided; 1 1/4 pounds boneless, skinless chicken breast tenderloins; 1 1/2 teaspoons kosher salt , divided; 3/4 teaspoon freshly ground black pepper , divided, plus more for garnish; 1 large sweet onion , thinly sliced (about 3 cups ); 1 tablespoon light brown sugar; 2 tablespoons all-purpose flour; 1 cup beef broth; 2 tablespoons Worcestershire sauce; 2 teaspoons chopped fresh thyme , plus more for garnish; 4 ounces Swiss cheese, shredded (about 1 cup)