Prepare the brine by bringing water, salt, brown sugar, thyme, cloves, peppercorns, star anise and lemon peel to a boil in a large pot, dissolving the salt and sugar. Remove from heat and allow to chill completely. Place the pork belly in brine for one hour per inch of meat.
Preheat oven to 300°F. Sear pork belly fat-side down over medium heat, then place the pork belly in a 5 L roasting pan. Add orange juice, orange, onion, celery and carrot into the roasting pan and cover with chicken stock. Cover the roasting pan with aluminium foil and roast in preheated oven until pork belly is tender, about 3 hours. Remove from heat and let pork belly cool in the pan with all the juices. Strain braising liquid, stir in miso paste, and set aside.
When ready to serve, preheat oven to 400°F. Cut pork belly into equal portions and place skin-side down in an oven-safe skillet. Place in preheated oven until pork belly is heated through, about 30 minutes. Remove from heat and pour in ½ cup braising liquid/miso mixture. Slowly stir in butter to create a glaze. Baste pork until the glaze reaches a syrup consistency.