Place each chicken-breast half between 2 sheets of wax paper and pound with a meat mallet until about 3/4 inch thick.
Heat the oil in a large skillet over medium-high heat and saute the chicken until golden, about 3 minutes per side. Add the apple slices, apple juice, onion, garlic, thyme, and salt. Cover and simmer 6 to 8 minutes or until the chicken is fork-tender. Remove the chicken, apple slices, and onion to a serving platter and keep warm.
Bring the sauce to a boil for about 5 minutes or until slightly reduced. Whisk in the mustard. Pour the sauce over the chicken and serve.