Fresh and with an astringent taste, I can't imagine a big bowl of nabe without them. They should be very lightly cooked so that they retain their crunchy texture and flavor, so they are added to dishes last. The Chinese call these tong hao and incorporate them in stir fries. They're also key to many Taiwanese, Korean, and Vietnamese dishes.
You can find them at your local Asian market. Select greens that are fresh and firm, bright green in color, with no black spots.
Some recipes to try:
Steamboat, Hot Pot, Shabu Shabu: Like Fondue, But Better
Shungiku Salad with Sesame Soy Dressing - Country Living
(Image: Kathryn Hill)