Meanwhile, preheat a grill and lightly oil the grates.
Halve and pit the peaches. Run a sharp knife along each peach's seam to halve them. Remove the pit and brush each cut side with olive oil.
Place the peaches cut-side down on the grill and cook undisturbed until grill marks appear, 4 to 5 minutes.
Flip the peaches and cook until tender, 4 to 5 minutes, until the skins are charred and the peaches are soft.
Roll your thighs in your prepared onion sauce. You can use a large casserole pan, or a sheet of foil on a cookie sheet. (Fewer dishes!) Save the leftover sauce for drizzling.
Place thighs on the grill for approximately 5 minutes; flip chicken and allow it to cook for another 5 minutes.
Check the temp! Cook chicken until it reaches 165°F, you may need to cook another 4 to 5 minutes on each side and recheck.