Remove the pork loin from the refrigerator well in advance so the meat comes to room temperature. Season the meat all around with Celtic sea salt and coarsely ground black pepper. At the same time, bring a pot of water to a boil and cook the halved Brussels sprouts for four to five minutes until just tender. Drain the vegetables and let them drain well.
Heat the ghee or lard in a roasting pan or sturdy skillet over medium-high heat. Cook the pork loin for about ten to twelve minutes until browned on all sides, with the inside still light pink. During cooking, add the crushed garlic clove and fresh rosemary to the fat for extra flavor.
Remove the pork loin from the pan, loosely wrap the meat in aluminum foil, and let it rest. Then increase the heat under the same pan to high. Add the pre-cooked Brussels sprouts to the remaining aromatic fat and cook them for three to four minutes, searing them until they have crispy, golden-brown edges all around.
Remove the rested pork loin from the foil and slice the meat with a sharp knife into equal pieces. Divide the cooked Brussels sprouts among plates and arrange the pork slices alongside.