Make the brine. Fill a large pot with 1 gallon water. Add the garlic, fennel seeds, juniper berries, peppercorns and thyme. Bring to a boil, then reduce the heat and simmer 10 minutes. Remove from the heat, whisk in the salt and brown sugar and let cool to room temperature.
Brine the pork. Put the pork belly in a large roasting pan and pour the brine over it. Cover and refrigerate 24 hours.
Make the rub. Toast the fennel seeds, juniper berries and peppercorns in a medium skillet over medium heat until fragrant, 2 to 3 minutes. Pulse a few times in a spice grinder until coarsely ground. Transfer to a small bowl and stir in the nutmeg, thyme and garlic.
Spice the pork. Remove the pork belly from the brine and pat it dry. Spread the rub on the flesh side of the pork belly, using just what adheres; you may have some rub left over. Put the pork belly skin-side up on a large rimmed baking sheet and refrigerate, uncovered, 24 hours to dry it out.
Roll the pork. Preheat the oven to 475 degrees F. (If you have a convection setting, use it; preheat to 450 degrees F.) Flip the pork belly so it's skin-side down. Starting at a long side, roll into a cylinder.
Secure the roll with kitchen string: Tie separate pieces of string around the roll, about 3/4 inch apart.
Prick the pork all over with a skewer; this helps render the fat so the skin gets crisp.
Roast the pork. Put the pork belly on a rack set on a rimmed baking sheet. Tuck 4 upside-down spoons around the pork to keep it from rolling around. Roast 30 minutes, using a baster to baste the pork with the drippings and rotating the pan every 10 minutes. Turn the oven temperature down to 350 degrees F (325 degrees F for convection) and continue roasting, rotating the pan every 15 minutes, until the pork registers 155 degrees F on a thermometer, about 1 1/2 more hours.
Slice the pork. Let the pork rest 30 to 40 minutes, then cut off the string and slice about 3/4 inch thick.