Cut off and discard root end of bok choy, leaving stalks with leaves. Cut green leaves from stalks and cut the leaves into 1-in. slices; set aside. Cut white stalks into 1-in. pieces.
In a large cast-iron or other heavy skillet, cook bok choy stalks in butter and oil until crisp-tender, 3-5 minutes. Add the radishes, shallot, lemon pepper, salt and reserved leaves; cook and stir until heated through, 3 minutes.