Bring a large pot of salted water to the boil. Add the potatoes and cook for 12–15 minutes until just tender. Drain, cool slightly and slice.
Heat olive oil and butter in a large frying pan over medium heat. Add the leek and cook for 4–5 minutes until soft. Add bacon and garlic and cook for a further 2–3 minutes.
Sprinkle over the flour and cook for 1 minute. Gradually stir in the chicken stock, then milk and cream, simmering gently until thickened and creamy.
Stir through the chicken, parmesan and tasty cheese until melted and smooth. Season generously with salt and pepper.
Preheat the oven to 200°C (fan). Lightly grease a baking dish. Layer half the chicken mixture, then half the potatoes, season lightly, and repeat.
Cover with foil and bake for 30 minutes. Remove the foil, scatter over extra cheese and bake or grill until golden and bubbling.
Rest for 5 minutes, then serve with a simple green salad or vegetables.
