Mix together the tarragon, mustard, garlic, lemon juice and some salt and pepper and stir so they are combined.
Place the chicken thighs in an oven dish and pour over the tarragon-lemon mixture and ensure all of the thighs are well coated. Allow to marinate for 30 min to 2 hours, as far as possible.
When ready to cook, preheat the oven to 375F/190C.
Bake the chicken in the oven, skin side up, for approx 30-35 minutes until lightly crisp and browned on the top and the chicken is cooked through (time will vary a little depending on how large and tightly packed your chicken is).
Serve hot, suggested over rice with the cooking juices poured over the top.
Note: Prep time is only 5 mins, but ideally you want some marinading time, 30 min or more. If not possible, it will still work without, you will probably just have less depth of flavor.