Swiss Chard with Horseradish

Cut out stems and center ribs from chard, discarding any tough parts (see Cooks' notes), then chop. Wash leaves well, then cut in half crosswise (leave whole if on the smaller side). Heat oil in a wide heavy pot (5- to 7-quart capacity) over medium heat until shimmering, then cook onion, stirring occasionally, until it begins to soften, about 5 minutes. Add chard stems and ribs, the water, and 1/2 teaspoon salt, and cook, covered, until just tender, 8 to 10 minutes. Increase heat to medium-high. Add leaves in handfuls, turning with tongs and adding more as previous batches wilt, until all leaves have been added to pot. Cover with lid, then reduce heat to medium and cook, stirring once or twice, until just tender (do not overcook), 2 to 3 minutes. Remove from heat and stir in horseradish.