If you can't find watercress, one (10-ounce) package of prewashed baby spinach will be just as delicious. For milder onion flavor, rinse onion slices, pat dry, and then add to salad.
Remove and discard tough stems from watercress. Place watercress, oranges, and onion in a large bowl.
Whisk together oil and next 6 ingredients. Drizzle over watercress mixture, and toss to coat. Sprinkle with blue cheese, and serve immediately.
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