Preheat the oven to 200°C fan 180°C gas 6. Bring a large saucepan of water to the boil, then add the potatoes and boil for 10 minutes. Drain and leave to cool slightly, then thinly slice.
Meanwhile, heat 2 tablespoons of the olive oil and fry the sliced turkey breast steaks for 5 minutes until golden. Add the roughly chopped leeks, mushrooms and garlic. Cook for a further 5 minutes.
Make up the instant white sauce mix, following pack instructions. Transfer the turkey and vegetable mixture to an ovenproof dish, pour over the sauce and stir through. Top with the potatoes and brush with the remaining olive oil. Sprinkle with the dried mixed herbs and bake for 30 minutes until golden.
Cook's tip: you can use leftover turkey for this recipe too.