Preheat the oven to 425 degrees. Place the cast-iron skillet in the oven while it is preheating.
While the oven is preheating, dice the potatoes and onion into small pieces.
Put them in a small bowl and toss with avocado oil, salt, and pepper. Set aside.
Once the oven is preheated, remove the cast iron skillet from the oven.
Place the whole chicken in the middle of the skillet. Pour the melted butter over the top of the chicken and sprinkle with salt, pepper, and thyme.
Pour the potatoes and onions around the outside of the chicken.
Cook in the preheated oven for 30 minutes. Turn the oven down to 350 degrees and continue to roast until the potatoes are crispy and chicken reaches an internal temperature of 165 degrees.
Remove the skillet from the oven and allow it to rest for about 10 minutes before carving into the chicken.
Serve with sauerkraut or a mixed greens salad with homemade ranch dressing.