To reheat pork tenderloin while retaining its moisture and tenderness, start by slicing the meat into even pieces to ensure uniform heating. Preheat your oven to 180°C (350°F). Place the slices in a baking dish and cover them with foil to prevent them from drying out. You can add a splash of chicken stock or a drizzle of olive oil for added moisture. Heat in the oven for about 15 to 20 minutes, or until the internal temperature reaches 63°C (145°F). Alternatively, you can reheat individual slices in a frying pan over medium heat for 2-3 minutes on each side. Serve with your favourite side dishes, and enjoy the delightful flavours of your perfectly reheated pork tenderloin.