Pit the peaches, slice into ¼" thick slices, set aside.
In a small pot, whisk together melted butter, onion, Southern Comfort, honey, parsley, and chili flakes. Season with salt and pepper to taste.
Bring sauce to a boil, turn heat down and simmer for 5 minutes. Set aside half of the sauce for serving, bring the remainig sauce to the grill to baste with.
Season chicken breasts with salt and pepper.
Grill chicken for about 6 minutes on each side, or until cooked through (Time will depend on temperature and thickness of chicken), basting with sauce every few minutes
Once you’ve turned the chicken for the first time, start grilling the peach slices, also basting with the sauce. They’ll cook a lot faster than the chicken, so keep an eye on them!
Once chicken is cooked through, transfer to plates, top with grilled peaches, and drizzle with reserved sauce.