Preheat your oven to 375 °F (191 °C). Heat ½ tablespoon of butter in a large skillet over medium-high heat. Once the butter is melted and bubbling, add the chicken skin-side down. Season the side facing up with salt and pepper. Leave the chicken to sear undisturbed for 5-7 minutes, or until the skin side is crispy and you can easily flip the thighs over using a pair of tongs.
After flipping over the thighs, allow them to sear for an additional 3-5 minutes on the side without skin. Set the thighs aside and drain out half of the fat out of the pan.
Melt the remaining butter in the pan over medium heat. Once melted, add the onion and saute for 5 minutes or so, or until soft. Add the garlic and half of the thyme and stir until fragrant.
Add the dijon mustard, stone ground mustard and stir. Add the broth, wine and honey and whisk together. Bring the mixture to a boil, then reduce the heat so the liquid is simmering. Allow to simmer for 30-45 seconds, or until the liquid begins to just slightly thicken. Nestle the chicken back into the pan and top with the remaining thyme.
Place in the oven and roast until the internal temperature of the chicken reads 165 °F (74 °C) with a meat thermometer. The time will depend on how large your chicken thighs are, but aim to check them around 25 or 30 minutes and then continue cooking until they reach the appropriate temperature.
Remove the pan from the oven and place on the stovetop. Set the chicken thighs aside once more and whisk the milk slowly into the pan. Heat the pan over low heat and stir. Bring the mixture to a simmer and stir until the sauce reaches your desired thickness. Place the chicken thighs back in the sauce and spoon some of the sauce on top. Season with salt and pepper and top with additional thyme.
Serve the chicken thighs alongside your favorite side dishes.