Preheat oven to 400 degrees Fahrenheit.
Make the mustard rub in a small mixing bowl: olive oil, 1 tablespoon whole grain mustard and salt and pepper to taste. Dry the chicken thighs before covering in the rub. Leave in the bowl while you make the mustard sauce.
In a medium size mixing bowl, whisk together the three mustards, honey and chicken stock. Depending upon the consistency you want, add more or less chicken stock. Set aside.
Heat olive oil in a large oven-proof skillet to medium heat. Add the chicken thighs, skin side down, and merely sear both sides until golden brown (about 2-3 minutes on each side). Upon flipping to the second side, cover each thigh in the three mustard sauce and add sprigs of rosemary to the pan in between the thighs.
Place into the oven (uncovered) until completely cooked through (175 degrees Fahrenheit, internal temperature of the chicken). Approximately 25-30 minutes, but check at 20 minutes.
Serve immediately and enjoy the tender, luscious flavor.