Trim the bottom of the radicchio head and cut the head into wedges that are about 1½ inches (4 cm) thick on the widest side, keeping the core intact. Put on a plate and rub the oil all over the wedges, keeping them as wedge-like as possible, and sprinkle with salt (about ½ teaspoon total) and several grindings of pepper. Heat a large skillet or sauté pan over medium-high heat. When a drop of water on the surface evaporates immediately, it’s hot enough. Add the radicchio, cut side down, in a single layer (you might need to do this in two batches) and cook without disturbing them for 2 to 3 minutes, until deeply browned. Using tongs or a thin metal spatula, turn the wedges and brown the other cut side. Remove to a platter or serving bowl and serve.