INGREDIENTS   3 tablespoons unsalted butter 2 pounds onions, sliced into thin half-moons   Kosher salt and freshly ground black pepper 4 garlic cloves, sliced 2 small sprigs thyme, leaves only   4-inch sprig rosemary 2 cups chicken broth, divided 1 tablespoon balsamic vinegar 2 tablespoons Dijon mustard 3 pounds boneless skinless chicken thighs [...]