Make the quick pickles: Place the carrots and daikon in a heat-safe bowl or jar. In a small microwave-safe bowl, combine the rice vinegar, water, sugar, and salt. Microwave just until warm, about 30 seconds, then stir until the sugar dissolves. Pour over the vegetables and set aside while preparing the sandwiches.
Season the chicken: In a medium bowl, combine the chicken, fish sauce, soy sauce, brown sugar, lime juice, and garlic. Toss until the chicken is evenly coated.
Make the spicy mayo: Stir the mayonnaise and sriracha together in a small bowl. Use less sriracha for a mild family-friendly spread, if preferred.
Cook the chicken: Heat the oil in a large skillet over medium-high heat. Add the chicken in a single layer, working in batches if needed. Cook, turning occasionally, until browned and the thickest pieces reach 165°F (74°C), about 8 to 12 minutes total.
Toast the bread: Lightly toast the split baguettes or rolls until the cut sides are crisp and warm.
Assemble the sandwiches: Spread spicy mayo on the cut sides of the bread. Divide the cooked chicken among the rolls, then add drained pickled vegetables, cucumber, cilantro, and jalapeño. Close the sandwiches and serve right away.