1. Combine first 5 ingredients in a small bowl, stirring with a whisk.
2. Place watercress in a large bowl. Drizzle half of vinaigrette over watercress; toss gently to coat.
3. Divide watercress mixture evenly among 4 salad plates; top each serving with 4 egg quarters. Drizzle salads evenly with remaining vinaigrette; sprinkle evenly with salt and pepper.