How do you get a sear on chicken breast without overcooking it?
You have two options:


A very hot sear on both sides, then move it to a cooler oven (about 350 to 400°F (175 to 205°C) or indirect heat on a  grill) to finish.
Cook it in an oven at a lower temperature (350 to 400°F again) until the middle is cooked through (or a few degrees off for carry-over), then sear on a hot pan or grill.


I can't remember who did it, but there was some testing for how thick the overcooked portion was on steaks, and they found it better to sear after.  I would assume the same would be true for poultry.