Ideally, you would use the elongated Treviso radicchio, now starting to become available in the States, but this recipe is also excellent with the more common round radicchio.
Put the olive oil in a heavy-bottomed braising pan and place over medium-high heat. When the oil is hot, put in the pork. Brown the pork on all sides, then transfer to a platter and season with salt and pepper.
Turn the heat down to medium and put in the onion. Sauté, stirring occasionally, until it turns a rich golden