Balsamic vinegar works to balance the bitterness of radicchio. This makes a fine alternative to a salad and goes especially well with rich, creamy dishes like gratins. It's also nice with soem crumbled feta, blue cheese or grated Parmesan and toasted hazlenuts or almonds on top. It's also a useful ingredient: Chop it coarsely and stir it into risotto toward the end of cooking or toss together with fresh pasta. Combine with sliced steamed green beans or cooked cannellini beans.

4 small or two large heads radicchio (about 1 pound)
2 tablespoons extra virgin olive oil
1/4 cup balsamic vinegar
1 tablespoon brown sugar or honey
Salt and pepper to taste

Heat grill or broiler to moderately high heat and put rack 4 inches from heat source.
Core the radicchio and then halve or quarter them, depending on their size. Rub or brush the radicchio with the olive oil taking care to keep the wedges intact. Combine the sugar and vinegar in a small bowl until dissolved.
Put the radicchio wedges on the grill or on a broiler pan, cut sides toward the heat. Grill or broil for a minute or two, then carefully turn and brush, or drizzle, with the balsamic vinegar mixture. Cook until just starting to crisp and char around the edges, another couple of minutes. Transfer to a plate and sprinkle with salt and pepper. Serve hot or at room temp.

Source: How to Cook Everything Vegetarian, by Mark Bittman