Put all the yogurt, onion, ginger, garlic, cilantro, garam masala, cumin, coriander, salt, fennel seeds, chile powder, pepper, oil, and lemon juice into a food processor and blend until smooth. Pour the mixture into a large bowl and stir in the lamb cubes. Mix well to coat the lamb cubes. Cover the bowl and leave to marinate in the refrigerator for as long as possible, preferably overnight. Remove the bowl from the refrigerator and let it come to room temperature an hour before cooking.
Preheat the grill to medium heat or preheat the oven to 400 degrees F.
Thread the lamb cubes onto the skewers leaving a space between each piece and place under the grill. Cook for around 5 to 6 minutes, turning halfway through cooking, or until tender. Alternatively, cook in an oven for 8 minutes. Baste the skewers with the melted butter and cook for another minute or 2 for slightly pink meat, or longer if you prefer the lamb well done.
Divide the lamb pieces into five equal portions and place each portion in the center of a warmed tortilla. Top with a handful of the sliced vegetables and a spoonful of chutney. Roll up and serve.