Mix garlic powder, onion powder, paprika, cayenne, salt, and pepper in a small bowl. Reserve 1 tsp of this seasoning mix for the gravy.
Season the chicken thighs all over with the seasoning mixture, leaving 1 teaspoon reserved for the gravy.
Sauté onion + garlic: Heat 2 Tbsp oil in a large cast iron skillet over medium-high heat. Sauté onion 2 minutes until soft. Add minced garlic and cook 30 seconds until fragrant. Remove onion/garlic to a plate.
Sear chicken: Add a little more oil if the skillet looks dry. Sear chicken skin-side down first until the skin is crispy and deep golden, then sear the other side. Remove chicken to a plate. Carefully drain off most of the fat, leaving about 1–2 Tbsp in the skillet.
Make the gravy: Reduce heat to medium. Add butter. Once melted, whisk in flour and cook 30–60 seconds. Slowly whisk in milk until smooth. Stir in Better Than Bouillon and the reserved 1 tsp seasoning. Bring to a gentle simmer.
Simmer chicken + mushrooms: Add chicken back to the skillet, then add the cooked onion/garlic and mushrooms. Bring to a light boil, then reduce heat to low, cover, and simmer 30 minutes. (Check the gravy—if it thickens too fast, add a splash of milk.)
Scrape and smooth: Remove chicken briefly. Scrape up browned bits from the bottom of the skillet and stir into the gravy. If gravy is too thick, thin with a splash of milk. Return chicken to the skillet.
Steam bok choy (one-pot): Lay bok choy on top of the chicken (it’s okay if a little sinks). Turn heat to medium-high until it starts bubbling, then reduce to low, cover, and cook about 6 minutes, until bok choy is tender.
Serve: Taste the gravy and adjust salt, pepper, or onion/garlic powder if needed. Serve hot.