Preparation Soak onion in cold water to cover 15 minutes. Whisk together mustard, zest, juice, and salt and pepper to taste. Add oil in a slow stream, whisking until emulsified. Drain onion well and toss together with radicchio, cabbage, tomatoes, and enough dressing to coat. Toss salad with sugar and salt and pepper to taste.
Preparation Soak onion in cold water to cover 15 minutes. Whisk together mustard, zest, juice, and salt and pepper to taste. Add oil in a slow stream, whisking until emulsified. Drain onion well and toss together with radicchio, cabbage, tomatoes, and enough dressing to coat. Toss salad with sugar and salt and pepper to taste.