How to Make It
In a bowl, whisk the vinegar, lemon juice, preserved lemon, minced shallot, fish sauce and sugar. Whisk in 1/4 cup of the oil. Stir in the chopped parsley and season the vinaigrette with salt and pepper.
Season the trout with salt and pepper. In a large skillet, heat 2 tablespoons of the oil until shimmering. Add 3 trout fillets skin side down and press with a spatula to flatten. Cook over high heat until the skin is crisp, 3 minutes. Flip the fillets and cook until the fish is just white throughout, about 30 seconds; drain on paper towels. Wipe out the skillet. Repeat with the remaining 2 tablespoons of oil and 3 trout fillets.
Set the trout on plates, skin side up, and top with the watercress. Spoon some of the vinaigrette over the fish and watercress. Pass the remaining vinaigrette at the table.