Heat the oven to 425°F and line a sheet pan with parchment paper.

Soak the potatoes in cold water 15 minutes, drain, and pat completely dry.

Toss with oil, cornstarch and salt, spread in a single layer, and bake 35 to 40 minutes, flipping once, until deep golden and crisp.

Stir the Greek yogurt, cucumber, lemon juice, dill and a pinch of salt together in a small bowl to make the tzatziki, then chill it until needed.

While the fries bake, mix the ground lamb with the grated onion, garlic, oregano, cumin and coriander in a bowl.

Cook the meat in a skillet over medium-high heat, breaking it into small crumbles, 6 to 8 minutes, until no pink remains.

Pile the hot fries onto a platter and top with the cooked gyro meat.

Scatter the tomato, red onion and feta over the top, then finish with dollops of cold tzatziki just before serving.