What is the difference between bánh mì gà xé phay and bánh mì gà? Bánh mì gà uses lemongrass marinated chicken thighs grilled over high heat. The flavor comes from the char and the lemongrass marinade. Bánh mì gà xé phay uses poached chicken shredded and dressed in fish sauce and lime. There is no char, no marinade, no caramelization. The flavor comes entirely from the poaching liquid and the dressing. They are completely different sandwiches that happen to use the same protein.
Can I use a rotisserie chicken instead of poaching? Yes. Shred the rotisserie chicken while it is still warm and dress it immediately with the fish sauce, lime juice, sugar, garlic, and white pepper mixture. Skip the poaching liquid in the dressing and substitute 3 tablespoons of warm water or light chicken stock. The result is slightly less refined than the poached version but correct for a faster preparation.
Why does the chicken need to be dressed while warm? Warm chicken has open muscle fibers that absorb liquid. As the chicken cools the fibers contract and the dressing sits on the surface rather than penetrating the meat. Chicken dressed warm absorbs the fish sauce and lime throughout every strand. Chicken dressed cold produces a wet, underflavored result.
How do I know when the chicken is cooked through? The internal temperature should reach 82°C (180°F) measured at the thickest part of the thigh away from the bone. At this temperature the meat pulls cleanly from the bone and shreds without resistance. A lower temperature produces meat that is safe to eat but resists shredding. Use an instant-read thermometer for accuracy.
Can I make this ahead? Yes. The dressed shredded chicken keeps refrigerated for up to 2 days. The flavor improves as the dressing penetrates further overnight. Bring the chicken to room temperature before assembling. Cold chicken straight from the refrigerator produces a sandwich where the bread absorbs moisture from the cold filling and softens too quickly.