Mix marinade ingredients together. Reserve half of marinade in a covered container and refrigerate it for later.
Place remaining marinade in a gallon sized ziplock bag along with the tenderloin. Press out the air and seal the bag. Make sure the tenderloin is completely coated with the marinade. Refrigerate overnight.
Preheat oven to 350 degrees. Bake tenderloin for 20 minutes, then spoon reserved marinade over the top. Bake an additional 10 minutes or so until the meat is cooked to your desired doneness.
Remove from oven. Let rest at least 10 minutes before slicing.
Place one or two slices of tenderloin, a little sauce, and cilantro onto one half side of each piece of flatbread. Fold the bread over to cover and secure with a toothpick. Serve warm.
Variation: Slice tenderloin crosswise into 1/8" thick slices. Cover with half of marinade and let soak for 30 minutes. Thread meat onto bamboo skewers. Grill 1-2 minutes on each side. Warm the remaining marinade and spoon onto the meat when making the sandwiches.