In a small bowl mix the olive oil, garlic, salt, peppercorns, and crumbled bay leaves and rub the mixture all over the meat. Put the pork on a large dish or in a large plastic bag and cover the dish or tie up the bag and leave in the refrigerator if you've got steeping time; otherwise - if you're about to start cooking it - just leave it out.
Preheat the oven to 400 degrees F. Line a roasting pan with the onion. Strew over the onion 10 whole bay leaves. Place the pork, including its marinade, on top and the bones all around, if they fit and if you've got them. Roast in the oven for about 1 3/4 hours or to an internal temperature of 150 to 155 degrees F on an instant-read thermometer, basting regularly.
Remove the pork, scraping burnt bits off, to a plate or carving board and let it sit. On the stove at moderate heat, pour the wine and 1/2 cup boiling water over the bones, bay, garlic, and onion. Let it bubble up and reduce by about a third, and then remove the bones gingerly and strain the liquid contents into a saucepan. Heat, correct the seasoning, and add liquid as you like to make a good, thin, not-quite gravy.
Carve the loin, put the slices on a big warmed plate, sprinkle with salt, and pour over a little of the juice-gravy, then tent with foil and leave in the turned-off oven while you eat the starter. It is a bit prinky, I know, but it will look fabulous if, when you take it out, you arrange, Napoleonically, some more bay leaves around the edges of the dish with the bay-scented pork.
Chef's Note: If you've got time, leave the pork in its marinade-rub for 12 or even 24 hours. But otherwise, just do the necessary when you get home in the evening. By roasting the pork at 400 degrees F you can accommodate both croutons and meat. You wan the loin boned with a very thin layer of fat still on and tied at regular intervals. That's why I go to the butcher. And ask him to chop the bones and give them to you to take home while he's about it.