Combine the first 7 ingredients in a bowl, and stir well with a whisk. Cover and chill.
Combine the celeriac and 2 tablespoons dressing.
Combine the celeriac mixture, lettuce, and watercress. Drizzle remaining dressing over salad; toss gently to coat. Spoon 1 cup salad onto each of 6 plates; sprinkle each serving with about 1 teaspoon walnuts.