In a zip-top bag, combine the shrimp, tamari, vinegar, lime juice, sugar, garlic, ginger, and red pepper flakes. Shake to mix. Press the air out of the bag, seal the top, and marinate the shrimp in the refrigerator for at least 1 hour or up to 4 hours.
Heat a nonstick sauté pan on medium. Pour the shrimp mixture into the pan and cook until the shrimp is heated through, 3 to 5 minutes. Pour the mixture into a bowl and set it aside.
Wipe out the pan, place the flatbreads inside in a single layer, and warm them on medium until heated through, 1 to 2 minutes per side.
Spread the peanut butter over the bubbly side of the warm flatbreads. Using tongs or a slotted spoon, remove the shrimp from the marinade and arrange it on one half of each spread bread. Toss the cucumber, lettuce, scallions, and fresh herbs in the marinade until well coated. Using tongs or a slotted spoon, remove the mixture from the marinade and arrange it over the shrimp. Scatter on the sesame seeds (if using). Fold the other half of each flatbread over to cover the toppings, and serve.
Tip: If you have a choice of shrimp, go for large or medium instead of jumbo. The smaller size fits better in this flatbread sandwich. Having trouble finding naan style flatbread? Flatbread is available by special request at your local Panera Bread bakery-cafe.