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)