Place a rack in the middle of the oven and preheat it to 450 degrees F (230 degrees C).
Place the Potato (1.5 pound), Onion (1), Olive Oil (2 tablespoon) and Water (1/2 cup) in a large oven-safe skillet. Season all over with Kosher Salt (to taste). Crack Ground Black Pepper (to taste) over top to taste. Toss to combine.
In a medium bowl, stir together the Mayonnaise (1/4 cup) and the Dijon Mustard (2 tablespoon). Add the Bone-in, Skin-on Chicken Thigh (4) and toss to coat on all sides.
Season all over with Kosher Salt (to taste) and Ground Black Pepper (to taste). Nestle the thighs on top of/in between the potatoes. Crack more pepper over top if you wish.
Transfer pan to the oven uncovered and cook for 45 minutes or until evenly golden, checking after 30 minutes to ensure everything is browning evenly — if the onions are getting too dark, add a few more tablespoons of water to the pan.
Sprinkle Tarragon (2 tablespoon) all over the roasted chicken and potatoes. Transfer chicken to a platter.
Toss potatoes with herbs and remaining juices in the pan. Taste. Adjust with more salt or pepper if necessary. Spoon potatoes around chicken on the platter, then serve.