Heat oil in a large ovenproof skillet over medium-high heat. Season chicken liberally with salt and pepper. Add chicken to the pan; sauté until well browned on both sides (about 2 min. per side). Remove chicken from pan; set aside. Reduce heat to medium and add garlic; cook, stirring frequently, until garlic is fragrant (2-3 min.). Add broth and syrup to the pan; bring to a boil, whisking pan to loosen browned bits. Cook for 2 min., stirring frequently. Add mustard, vinegar and mustard seeds; cook and stir for 1 min. Return chicken to pan; spoon mustard mixture over chicken. Move pan to a preheated 400° oven; bake until chicken is done (about 10 min.). Remove chicken from pan; tent loosely with foil and let rest for 5 min. Place pan with remaining sauce over medium heat; cook and stir until slightly thickened. Remove sauce from heat; whisk in butter. Adjust salt and pepper, if needed; stir in thyme. Drizzle chicken with warm sauce; serve with sweet potatoes, if desired.