Take yogurt in a bowl, add salt, chaat masala, garam masala powder, dried fenugreek leaves powder, red chilli powder, ginger garlic paste, ½ tbsp lemon juice and mustard oil and mix well.
Take fish fillets in another bowl, add salt, turmeric powder and remaining lemon juice and mix well.
Add the fish to this marinade and mix gently so that all the pieces and well coated. Cover and set aside in refrigerator to marinate for 1 hour.
Heat a non-stick grill pan. Insert 3 fish pieces on each skewer, keeping a little gap between each piece. Drizzle oil on the grill pan, place the fish skewers on it and cook on medium heat, rotating the skewers, till golden brown grill marks appear on both sides. Baste the fish fillets with butter occasionally.
Spread mint mayo sauce on each tortilla. Tear and place lettuce leaves, carrot and cucumber strips over them. Place grilled fish pieces in center and roll the tortillas. Place each roll on a paper.
Roll tightly covering only the bottom of the rolls and tie with a thread. Place the rolls on a serving platter and serve hot.