Prepare a water bath at 65 C or 150 F.
Trim the excess skin from the chicken thighs and season both sides generously with salt and pepper. Lay flat (don’t stack) in a 1 gallon ziplock freezer bag (make sure it is a freezer bag). Add thyme, butter, lemon zest, and fish sauce.
Seal using the water evaporation technique (see blog post).
Cook in water bath for 1.5 hours. Using tongs, remove from water bath. Remove the chicken from the bag, remove the aromatics, and drain the juices into a small pan. Let the chicken rest for about 10 minutes (or chill in and ice bath and then refrigerate for later use).
Heat about 1/4 cup of olive oil in a medium saute pan over medium-low heat (non stick works well). Cook the chicken in the pan skin side down until the skin is crispy, 7-10 minutes, then flip and cook for another 3-4 minutes on the other side.
To finish the dish, bring the juices the pan to a boil and reduce by about 1/2. Swirl in a 1/2 tablespoon of butter and a squeeze of lemon. Transfer the chicken to plates and spoon the sauce on top.