Combine the marinade ingredients in a container or zip-top bag. Remove half of the marinade to a separate container and reserve for the potatoes. Add the lamb to the remaining half of the marinade and marinate for 4-12 hours.
To make the pickled red onions, in a small saucepan, heat the water, sugar, vinegar, and salt. Place the onion in a heatproof bowl, cover with the vinegar mixture and cool to room temperature.
Preheat the oven to 425°F.
Heat a large, deep skillet over high heat with 2 tablespoons olive oil. Remove the lamb from the marinade, discarding excess marinade, and season with salt and pepper. Add the lamb to the skillet and sear until browned on all sides and cooked to medium-rare, about 4 minutes per side.
On a baking sheet, toss the potato wedges with the reserved marinade and roast until golden brown, about 20 minutes. Sprinkle with salt and pepper.
Combine all of the tzatziki ingredients in a small bowl.
Heat the naan directly on an oven rack for a few seconds
Fill the warm naan with lamb, romaine, tomatoes, tzatziki, pickled red onions and mint. Serve with potato wedges.