For the chicken and rub: In a medium bowl, mix the cumin, paprika, white pepper, thyme, onion powder, garlic powder and salt.
In a large bowl, add the chicken and pour the butter over it, massaging the butter into the skin and the cavity of the chicken. Generously coat the chicken with the rub and allow to rest for 30 minutes.
For the potatoes. Brush the bottom and sides of a large cast-iron pan with melted butter and set the remaining butter aside. Arrange the potato slices upright in a spiral around the pan (not flat like a gratin). Between every 8 to 10 slices, tuck in some garlic and randomly tuck in the red onion slices as well. Once done, brush the remaining butter over the entire dish and sprinkle with the red chile flakes and some salt. Cover tightly with aluminum foil or a lid.
Preheat the grill until extremely hot.
Put the chicken skin-side down on the grill over indirect heat. Immediately top the chicken with the cast-iron pan of potatoes and press down. Close the grill and cook the chicken and potatoes for 20 to 25 minutes. Uncover the potatoes, move the pan off the chicken and place it over indirect heat. Flip the chicken so it's skin-side up, place back over indirect heat as well and cook until a meat thermometer registers around 160 degrees F, 15 to 20 minutes more, drizzling the chicken with the honey about 5 minutes before it's done.
Set the chicken aside and gently tent with aluminum foil. Poke the potatoes with a fork and, if not tender, cook 5 to 10 minutes more with the grill closed. When ready to serve, drizzle the crema over the potatoes and sprinkle with the parsley. Serve with the chicken cut into parts.