Make a crosshatch score on the skin down to the meat, trying not to cut the meat, making the cuts close together. Rub the kosher salt and pepper all over, massaging well to get in the nooks and crannies.
Mix the marinade ingredients in a blender to emulsify. Poor over the pork belly and marinate 12 hours or more in a sealable container or Zip Lock bag in the refrigerator. Take out 30 minutes prior to roasting to bring to room temperature.
Preheat oven to 525°F.
Wash, peel and cut the carrots, onions and celery into 2-inch pieces and place in bottom of roasting pan. Add the whole garlic cloves, tossing with the vegetables and olive oil.
Place the marinated pork belly (save marinade), scored side up, on top and cook for 15 minutes until skin is bubbling and starts turning a golden color. Drop oven temperature to 325°F and cook for 1 1/2 hours.
To the reserved marinade and enough white wine to make 3 cups total liquid. Pour marinade/wine mixture over pork belly and roasting vegetables. Cook for 1 hour, or until the pork belly is fork tender and can easily tear apart.
Remove pork belly and keep warm (See Note 1). Place entire roasting pan on the stove top over medium heat. Sprinkle the flour over the vegetables and mash to create a thick paste. Cook for 2 to 3 minutes on a low heat (thin with chicken stock if needed), season to taste, then strain and press solids through a sieve into a gravy jug. Serve alongside roasted pork belly.
At this point you can allow the pork belly to cool and wrap in foil. Refrigerate overnight. Slice and pan fry to get crispy and serve with warm gravy.