Mix the flour, salt, pepper, garlic powder, and sage in a shallow pan. Dredge the chicken through the flour mixture.
Heat the oil in a large skillet over medium-high heat; add the chicken and brown each side 5 minute In a large mixing bowl, whisk together the cornstarch and milk, then add cider. Add the apple slices and cider mixture to the chicken; simmer, uncovered, until the apples are tender, about 20 minute The cider should be reduced by half of the original volume.
Place the chicken and apples on plates and top with the apple cider mixture.