Add the coriander to a spice grinder and pulse a few times until the seeds are just cracked. Set aside.
Sprinkle the chicken thighs all over with salt and pepper. Add the oil to a large cast-iron skillet and heat over medium-high. Once the oil begins to shimmer, sear the chicken, skin side down, until golden brown and crispy, about 5 minutes. Turn the thighs over and cook until brown, about 5 minutes more. Remove from the pan to a plate, skin side up, and set aside.
After the chicken is removed from the pan, add the cracked coriander and toast, stirring frequently, until fragrant, about 1 minute. Add the leeks and cook, stirring occasionally, until soft, about 7 minutes. Add the garlic and cook, stirring, until soft, about 2 minutes. Nestle the bok choy into the leeks and add 2 tablespoons of water. Bring to a simmer and cook until the liquid has reduced, about 3 minutes. Season with salt and pepper. Nestle the chicken, skin side up, into the vegetables and cook for a few minutes until the thighs are 165 degrees F in the thickest part. Remove from the heat and sprinkle with the chives. Serve right from the pan!