First, scrub the potatoes and cut into thin wedges. Place in a pan, cover with water and bring to the boil. Simmer for 5-7 minutes until cooked but not falling apart. Drain and put to one side.
Whilst the potatoes are cooking, wash and trim the leek, keeping as much of the green stem as possible (but discard any woody bits). Slice in half lengthways then cut into rounds. Slice the mushrooms.
Heat 2 tablespoons of water in the bottom of a medium-sized frying pan. Add the leeks and a pinch of salt and sauté on a medium heat for 5 minutes until it starts to soften. Add the cooked potato and sliced mushrooms, cover with a lid and sauté on a low heat for another 5 minutes.
Whilst the veggies are cooking, make the sauce by placing the cashew nuts, water, mustard, garlic and salt and pepper into the jug of a blend and blitzing to smooth.
Mix the topping ingredients together in a bowl. Pre-heat the grill to high.
Pour the sauce into the pan and stir to make sure everything is covered. Heat through for a few minutes without letting the sauce stick to the bottom of the pan. Turn off the heat and sprinkle the breadcrumb mix over the top.
Place the pan under the grill, making sure the handle is out of the way. Grill for 4 minutes or so until the breadcrumbs are lightly browned and crispy. Carefully remove the pan and serve.
