Preheat oven to 225 degrees. Cut the belly into 4 equal squares. Grind the fennel and coriander in a spice grinder or use a mortar and pestle. Combine with the salt and pepper and rub onto the pork belly on all sides. Heat the olive oil in a large saucepot or roasting pan to med-high heat. Sear the belly pieces on all sides. Remove from the pot, add the onions and saute for 2 minutes. Add the water, wine and bay, place the belly back in the pot with it. (If skin on, skin side should be up and not submerged in liquid). Place in oven uncovered and allow to simmer for 1.5-2 hrs or until tender. Remove from oven; create a sauce by skimming the fat from the cooking liquid. Strain the liquid into a saucepan, add the mustard and reduce by ¼.
For the lentils: