Make the pickled carrots and daikon: Place the carrots and daikon in a large strainer, sprinkle with salt and toss to mix. Let stand 20 to 30 minutes. Meanwhile, in a nonreactive saucepan, combine the water, vinegar, and sugar and bring to a boil. Remove from the heat and cool to room temperature. Rinse the vegetables briefly with cold water, then squeeze dry and transfer to a medium bowl. Pour the vinegar mixture over the vegetables and stir with a wooden spoon to ensure all the vegetables are well moistened. Let stand for 1 hour before serving.
Make the red onion: In a small bowl, combine onion, lemon, sugar, salt and pepper. Set aside to marinate 15 minutes.
Make the spicy mayonnaise: In a small bowl, combine mayonnaise, sriracha and lemon juice. Set aside. Make the sandwich: Slice the baguette in half the long way and spread each half with spicy mayonnaise. Layer watercress, onions, pickled carrots and daikon, smoked trout and cilantro. Slice sandwich in half and serve.
2 hours total to prep and pickle the carrots and diakon, 20 minutes to prep the red onions, and 10 minutes to compose sandwiches