Before you start, preheat oven to 220°C (or 200°C fan bake). Bring a medium pot of salted water to the boil.
Prep leek, kale, Parmesan, sage, potatoes and cannellini beans and set aside separately. Remove the tough inner stem from kale and roughly chop the leaves so you have about 1½ cups worth.
Cut the stalk off each mushroom and lightly squash down each mushroom with the palm of your hand. Whisk egg and flour together in a bowl and set aside. Toss seeded crumb, olive oil and salt together in a separate bowl. Working one at a time, dip mushroom into egg mixture and then coat in crumb mixture, pressing crumb firmly into mushrooms.
Arrange crumbed mushrooms on a lined oven tray and bake on upper oven rack for 20-25 minutes, until golden and crispy. Turn halfway through cook time.
Place kale and leek into a small roasting dish and pour over stock. Sprinkle over Parmesan and sage and drizzle with olive oil. Season with salt and bake on rack below mushrooms for about 25 minutes, until leeks are tender.
Cook potato and beans in pot of boiling water on high heat for about 15 minutes, until potato is tender. Drain well, return to pot with feta, butter and a pinch of salt and mash well, until smooth. Add a few tablespoons of milk if your mash is too thick. Season to taste with salt and pepper.
Serve mash and leeks topped with mushrooms. Top with sour cream.