In a medium size mixing bowl or glass measuring cup, combine the shallot, dijon mustard, whole grain mustard, honey, brown sugar and chicken stock, whisk to combine. Set aside until needed.
In a large oven-safe skillet, heat olive oil over medium-high heat. Generously salt and pepper both sides of the chicken, then sear both sides until golden brown, about 3 minutes on each side (note: if you use skin-on chicken thighs, place skin side down first). Pour mustard mixture over the top of the chicken, then add the rosemary sprigs in between.
Place in the oven and cook for 25-30 minutes or until cooked throughout. Remove and allow to cool for 5-10 minutes before serving.
*For a slightly creamier version, you can swap out the chicken stock for half & half or heavy cream.