Preheat an outdoor grill, grill pan or cast-iron skillet over medium-high heat. In a saucepan, melt the peanut butter over low heat. Stir in the chicken stock, tamari and brown sugar (or honey), then the chile, shallot, garlic, ginger and lime (or lemon) zest and juice. Gently simmer over low heat for 15 minutes.
Meanwhile, coat the chicken in the oil. Season with the turmeric, cumin, coriander, salt and pepper. Grill or broil, turning occasionally, for 12 minutes.
Splash a little water on the grill or grill pan and add the naan to crisp slightly. Melt the butter in a small saucepan or in the microwave and brush it onto the naan.
Slice the chicken and pile onto the naan. Top with the lettuce, scallion, cucumber and lots of sauce.