Wrap pitas in foil and warm in a low oven. Thinly slice the cooked lamb. In a small skillet over moderate heat bring the chicken stock to a boil, the reduce heat to low. Add the sliced lamb to reheat while keeping it moist.
To a second skillet over moderate heat add oil, 1 turn of the pan, diced tomatoes and scallions. Heat tomatoes and onions through, 2 minutes. Season with salt and pepper then turn off the burner.
Combine yogurt with cumin, coriander and mint.
To assemble, remove meat from warm broth with tongs and shake off excess liquid. Arrange a few slices of lamb down the center of the pita. Top with a few spoonfuls each of tomatoes and scallions and the yogurt dressing.