unsalted butter • boneless, skinless chicken breasts, trimmed of fat and cut crosswise into 1/4-inch slices • small onion, chopped fine (about 2/3 cup) • Table salt • medium cloves garlic, minced or pressed through garlic press (about 2 tablespoons) • red pepper flakes • chopped fresh thyme leaves • all-purpose flour • dry white wine • low-sodium chicken broth • broccoli (about 1 1/2 pounds), florets trimmed into 1-inch pieces (about 6 cups), stems discarded • penne pasta, ziti, cavatappi, or campanelle •