The only trick to making roast pork loin–aside, that is, from being patient during its long, slow spell in the oven—is knowing how to keep the pork loin from drying out. The answer lies in the kind of pork loin that your grandma's neighborhood butcher had readily available—meaning one with sufficient fat so the roast essentially bastes itself as the fat melts. It can be tricky to source this kind of pork nowadays. You may have to go to a few butcher counters before you find one that has a nice, thick fat cap attached. Persist. It’s worth the time and effort.