In a small bowl or measuring cup, combine all the ingredients for the vinaigrette except for the watercress and mix vigorously with a fork until well combined and emulsified. Stir in watercress and set aside.
Heat a few tablespoons of olive oil in a large, heavy skillet over high heat; once the pan is really hot, carefully place the trout fillets in the pan, skin side down. Sprinkle with salt and pepper and cook for about 3 minutes, or until the skin gets nice and golden, then flip the fillets and continue cooking for a further 3 to 5 minutes, depending on desired level of doneness.
Remove the fish from the pan and serve promptly, covered with a generous amount of the watercress vinaigrette.
Garnish with lemon slices and fresh watercress leaves, if desired.