Place the onions in a heatproof glass jar. In a saucepan, combine the vinegar, sugar and salt and bring to a gentle boil, and stir until the sugar and salt have dissolved. Pour the vinegar mixture over the onions. When cool, seal with a lid and transfer to the refrigerator. The onions will be pickled within 30 minutes, but the taste improves after 24 hours in the refrigerator.
Place a large, heavy-bottomed skillet on medium-high heat and add the oil. When the oil is glistening, add the onions and sauté until golden, about 5-7 minutes. Add the garlic and sauté for another 2 minutes, until golden. Add turmeric, chili powder, paprika, salt, and tomatoes and cook until a thick sauce begins to form, about 7-10 minutes. Add the chicken, ginger and red pepper and toss together. Cover with a lid and cook for 2-3 minutes, or until the red pepper is tender, but still crisp. Taste for salt, and add more, if needed. Allow to cool.
To the cooled chicken Jalfrezi, add the mint and toss well. Line each pita pocket with a lettuce leaf. Spoon the chicken Jalfrezi into the pita halves, dividing evenly. Add slices of pickled onions to each pita half.