Mix all the mustard crust ingredients together in a small bowl.
Preheat the oven to 450 F.
Using kitchen shears or a really sharp knife, cut the backbone off the chicken. Press on your hands to spread it out and flatten it.
Cut halfway into the joints connecting the legs and thighs. Then cut halfway into the joints of the shoulder under the wing.
With the chicken skin side down, rub half the mustard crust on it.
Put an ovenproof skillet onto a medium high burner and add the flattened chicken, mustard side down.
Spread the rest of the mustard onto the skin side.
Cook about 5 minutes, then put the skillet (and chicken) into the oven and cook until done, 30-40 minutes.
Let it rest a few minutes on a cutting board before cutting into pieces and serving.