Preheat the oven to 450 (yes, that is the right temp). Sprinkle the thighs on both sides with salt and pepper and let set while you prepare the dish. Using a 9x9 square or round oven proof dish (very important to use this size dish), mold heavy duty aluminum foil into the dish. Use at least two layers and make sure they criss cross so that your pan is well protected.
In a medium size bowl, mix the mustard, brown sugar and the vinegar together. Mix well and the resulting mix should have the consistency of molasses. You can add a little extra vinegar if it is too thick to stir easily (but you want it very thick).
Place your chicken pieces in the aluminum coated baking dish skin side down and pour half of the mix over the top of the chicken. Turn the chicken over and pour the other half over the skin side. Leave the skin side up. Use a spoon and make sure that each piece of chicken is very well covered and coated.
Once the chicken is coated well, place your dish on the center rack of your oven and bake for 40 mins. When done, you should have a nice gravy in the bottom of your pan. These are delicious served with long grain and wild rice and a green veggy with with the gravy spooned over the rice. SEE THE TIPS AND TRICKS SECTION BELOW