Preheat the oven to 375ºF.
Remove the pork loin from its packaging and pat dry with a clean paper towel in a baking dish that is large enough to hold the pork loin and whole apples. Make sure the pork loin is fat-side up.
Using a sharp paring knife, slice knife 1 to 2-inches into the pork loin all over, but preferably in a pattern of marks that are symmetrical both lengthwise and crosswise. For a large pork loin, make a 4x4 pattern with four slits across, and columns of four slits up and down the length of the pork loin.
Pack rosemaryand two or more slices of garlic in each slit, pushing them in and packing in as much as possible. Continue until garlic and rosemary are used up and all slits are filled. If you still have leftover rosemary, reserve for baking in the pan with the apples.
Generously drizzle the pork with oil, salt, and pepper on the top, bottom, and sides.
Roast the pork for 20 minutes and remove. Quickly and carefully place the whole apples alongside the pork loin. Lightly drizzle the apples with oil and juices that have accumulated in the bottom of the pan, and return to oven.
Roast for 10-20 minutes more, and check the internal temperature of the largest part of the pork loin. It should read 145ºF. Once done, remove from the oven, and let rest for at least 10 minutes.
To serve, keep the apples whole and slice around the core and seeds when eating. Dijon mustard makes an excellent accompaniment to this dish.