Ask your butcher to remove and reserve the skin and bone from the leg of pork. To make the scratchings, cut the skin into two pieces and score incisions through each piece. In a pestle and mortar, crush half the fennel seeds with half the coriander seeds, chile flakes, and a pinch of salt. Sprinkle the spice mix over the top and finish off with another good pinch of salt. Pour a little olive oil over the pork skin and bake in the oven, on a rack over a tray to catch the fat, in a preheated 425 degrees F (220 degrees C/gas 7) oven until very crispy.
Make crisscross incisions all over the pork, on both sides. Crush the remaining fennel and coriander seeds with another good pinch of salt. Sprinkle over both sides of the pork, and finish off with a grinding of black pepper. Place the pork and garlic cloves in a roasting tray. Cover with a few good lugs of extra-virgin olive oil. Place the sage and rosemary in a pestle and mortar and gently crush together until they are lightly bruised and have released their flavors. Add 4 tablespoons of olive oil. Squeeze the flavored oil over the pork and rub the herbs all over it. Cook in the oven in a preheated 400 degrees F (200 degrees C/gas 6) oven for about 4 to 4 1/2 hours. Any leftovers are great to use in sandwiches the next day.
