1. Preheat an oven to 400 degrees F.
2. In a large cast iron pan, add olive oil + butter over medium heat and fry the onions, carrots, and celery until soft.
3. Add the garlic and cook for another minute.
4. Mix well with the rest of the ingredients except the cornstarch slurry. Bring to a simmer for about five minutes. Finally, add the cornstarch slurry and let the dish sit while you prepare the biscuits.
5. In a large bowl, combine all the dry ingredients for the biscuits. Add in the buttermilk and combine to form the biscuit dough. Form 8 equal-sized balls of biscuit dough and place them on top of the chicken mixture. Transfer to the oven.
6. Bake for around 20-25 minutes. At about the 12-15 minute mark, coat the biscuits with a light brush of buttermilk. Remove when the biscuits are looking golden brown on top.