Boil the potatoes in a large pot until just cooked. Drain and let cool slightly, then slice thinly.

In a large pan, melt butter over medium heat. Add the leeks, ham, and garlic and sauté until fragrant. Sprinkle over cornstarch and stir gently. Slowly add chicken stock gel and milk, stirring until thickened. Then fold in cream, shredded chicken, cheese, and parmesan.

In a baking dish, layer half of the sliced potatoes. Spread half of the creamy chicken mixture over the potatoes. Repeat with remaining potatoes and chicken mixture creating two layers. Cover with foil.

Bake at 180°C (350°F) for 25–30 minutes.

Remove foil, top with extra cheese, and bake uncovered for another 10–15 minutes, or until the top is golden and bubbling.