Choose a heavy-bottomed or enameled cast-iron pot into which the pork can fit snugly. Put in the butter and oil, turn the heat on to medium high, and when the butter foam subsides, put in the meat, the side with the fat, if it has any, facing down. Brown the meat deeply all over, turning it when necessary. If you see the butter becoming colored a dark brown, turn the heat down a little.

Add salt, turning the meat to sprinkle all sides. Lightly crush the peppercorns with a mallet or meat pounder or even a hammer, then put them in the pot together with the bay leaves and vinegar. With a wooden spoon, quickly scrape loose browning residues from the bottom and sides of the pot, but do not let the vinegar simmer long enough for it to evaporate. Turn the beat down to low, cover the pot tightly, and cook, turning the pork occasionally, until the meat feels tender when prodded with a fork. If during this period the liquid in the pot becomes insufficient, replenish with 2 or 3 tablespoons water.

Transfer the pork to a cutting board. Let it settle for a few minutes, then cut it into slices about 3/8 inch thick or slightly less, and arrange them on a warm serving platter.

Tip the pot and spoon off most, but not all of the fat, and all the bay leaves. Add 2 tablespoons water, turn the heat on to high, and while the water bolls away scrape loose with a wooden spoon any cooking residues from the bottom and sides. Pour the pot juices over the pork and serve at once.