How to Make It
In a bowl, toss the chicken with the soy sauce, 1 tablespoon of mayonnaise and the shallot. Cover and refrigerate for 1 hour.
Meanwhile, in another bowl, mix the carrots with the pickled onions and their liquid and let stand for 1 hour.
Heat a grill pan. Grill the chicken and shallot slices over high heat until charred and the chicken is just cooked through, about 2 minutes per side.
Spread mayonnaise on the cut sides of the baguette, then spread with Sriracha. On the bottom half, layer the carrots, onions and cucumber, then top with the chicken and shallots. Lay cilantro sprigs over the chicken and close the sandwich. Cut crosswise into 6 pieces and serve.