Place each chicken breast half between 2 sheets of heavy-duty plastic wrap; pound to 1/2-inch thickness using a meat mallet or rolling pin. Sprinkle chicken evenly with salt and pepper.
Heat oil in a large nonstick skillet over medium heat. Add chicken; cook 6 minutes on each side or until done. Remove chicken from pan; keep warm.
Add garlic to pan; cook 1 minute, stirring constantly. Add broth and herbes de Provence; bring to a boil, scraping pan to loosen browned bits. Cook until broth mixture is reduced to 1/2 cup (about 3 minutes). Remove from heat; add butter and lemon juice, stirring until butter melts. Serve sauce over chicken. Garnish with thyme sprigs, if desired.