Trim Brussels sprouts and quarter lengthwise. In a large saucepan of boiling salted water boil sprouts 5 minutes, or until just tender, and drain in a colander. In pan cook onion in butter over moderate heat, stirring occasionally, until golden. Add sprouts and cook, stirring, 1 minute. In a small bowl stir together mustard and water and stir into sprouts with salt and pepper to taste.
