Combine all the ingredients in a bowl. Massage the rub all over the pork belly, then cover and refrigerate for 24 hours.
Preheat the oven to 325 degrees F. Remove the belly from the refrigerator. Coat a large straight-sided pan with olive oil and put over medium-high heat. Add the onion, celery, fennel, and garlic. Season the vegetables with crushed red pepper and salt, to taste. Cook the vegetables until they start to soften and become very aromatic, about 8 to 10 minutes. Add the wine and cook for 3 to 4 minutes. Stir in the mustard and 2 cups of the chicken stock. Taste to make sure it is delicious. Add the pork belly and toss in the thyme and bay leaves. Cover and put in the preheated oven. Braise the belly for 6 hours, checking occasionally. If the liquid level goes down add the remaining stock.
When the belly is done it will be very tender and succulent. Remove the lid and set the oven to broil. Broil the pork belly until it turns golden, about 3 minutes. Remove the pan from the oven and transfer the belly, from the braising liquid, to a cutting board. Cut the belly into 6 or 8 equal portions depending on your appetite. Arrange the pork on a serving platter and ladle the braising liquid on top.
It is also a great idea to rotate the pan a couple of times throughout the cooking process.