1. In a bowl, mix the vinegar with the olive oil, lemon juice, garlic and lemon zest. Season with salt and pepper.
2. In a large, deep skillet, bring 1 inch of water to a boil. Lightly oil a steamer rack and set it in the skillet. Carefully add the salmon fillets in a single layer. Cover and steam over moderate heat for 5 minutes. Strew the watercress over the salmon, cover and steam for 1 minute. Transfer the salmon and watercress to plates. Spoon the dressing over the salmon and serve.
Notes: One Serving - Calories 324 kcal, Total Fat 17.3 gm, Saturated Fat 3.3 gm, Protein 39 gm, Carbohydrates 4 gm