In a medium bowl, combine the fish sauce, sugar, minced lemongrass, soy sauce, garlic, shallot, and oil. Mix together. Add in the turkey breasts and coat each side with the marinade.
Cover with plastic wrap and place into the fridge for at least 6 hours, or preferably overnight.
Preheat grill to 200°C/392°F. Place marinated turkey breast onto the grill.
Cook for 5 minutes and carefully flip, cooking for another 5-6 minutes. Remove from grill and place onto a plate.
Cover the cooked turkey breasts with aluminum foil and let them rest for 10 minutes.
Assemble the banh mi sandwiches by spreading some butter on the bottom of the baguette and mayonnaise on the top half.
Add the grilled turkey breast, and top with do chua (pickled daikon and carrot), some cucumber, cilantro, and jalapeno (if you like it a little spicy).
Enjoy immediately. Grilled Lemongrass Turkey Banh Mi sandwiches may be tightly wrapped and stored in the refrigerator for up to 2 days. Another option is to keep the grilled lemongrass turkey and vegetables separately and assemble prior to eating.