Rub the pork belly all over with white wine. Use as much as you need to moisten the surface without getting it dripping wet. Do the same with the pork tenderloin.
Lay the pork belly skin side down on a flat surface, with the tenderloin down the middle. Sprinkle both very generously with salt and freshly ground black pepper.
Scatter the garlic all over the surface of meat, followed by the other filling ingredients so that the whole surface of the meat is well covered with herbs.
Roll the pork belly around the tenderloin, then truss it as tightly as you can manage.
Resting the porchetta
Set the porchetta on a baking sheet and leave it, uncovered, in the fridge at least overnight, up to two days.
Roasting the porchetta
Remove the porchetta from the fridge and let it come back to room temperature.
Place the porchetta on a rack in a roasting pan. Add a glassful of wine (or a mixture of water and wine) in the bottom of the pan.
Roast in a low (150C/300F) oven until you reach an internal temperature of 70C/160F, usually about 2-1/2 hours.
Remove the porchetta from the oven. Raise the oven temperature to 250C/475F, using convection if you have it. Pour a small glassful of water over the roast, then place in the oven. Roast for about 30 minutes, or until the skin is a deep golden brown and well blistered.
Serving the porchetta
Let the porchetta rest for at least 20 minutes before serving.
Serve warm or at room temperature. Slice thin as a sandwich filling or thick to serve as a main course.