Marinate the chicken: In a bowl, whisk together soy sauce, fish sauce, sugar, garlic, lemongrass, rice vinegar, and oil. Add the chicken thighs and toss to coat. Let marinate for 15–20 minutes while you prep the rest of the elements.
Cook the chicken: Heat a skillet over medium-high heat with a light coating of oil. Add the marinated chicken and cook until golden and cooked through, about 5–7 minutes per side depending on thickness. Rest the meat for a few minutes, then slice into thin strips.
Prepare the quick pickled onions (optional but recommended): In a small bowl, combine thinly sliced onion with a splash of vinegar and a pinch of sugar. Let sit while you assemble the rest, about 5–8 minutes.
Assemble the mayo spread: In a small bowl, mix mayo with lime juice, a splash of soy or fish sauce, and a touch of chili paste if you like a little heat.
Prepare the vegetables: Slice cucumber thinly, pat dry pickled onions, wash and dry cilantro, and prepare pickled carrot/daikon if you’re making a traditional banh mi version.
Build the sandwich: Slice the baguette lengthwise without cutting all the way through. Spread a generous layer of mayo, add the chicken, then top with cucumber, pickled onions, carrot/daikon, cilantro, and any additional herbs. Close and slice into portions.