Marinate chicken in lemon juice and garlic for 15 minutes. Add the tandoori paste and let marinate for as long as possible: 30 minutes to overnight.
Rub the chicken with vegetable oil, then cook: broil, grill, or brown in a nonstick frying pan. When the chicken is just cooked through, about 5 or 6 minutes per side (be careful not to overcook), transfer to a plate and let cool.
Cut the chicken into bite-sized pieces. Set aside.
Mix together mayonnaise and yogurt then stir in the mint. Set aside.
Brush the pitas on one side with water then heat either in a toaster or in a heavy nonstick frying pan. They should puff up, making it easy to split them to form a pocket.
Into each pocket place a few pieces of chicken, a spoonful of sauce, a sprinkling of cucumber, a little lettuce, more chicken and so on, until the ingredients are used up and/or the pitas can't hold any more. Garnish with onion, tomato and cilantro/mint then serve.