Trim and discard the woody bases of the king oyster mushrooms and wipe the remaining with a cloth to remove any dirt. Carefully drag fork prongs down the length of each mushroom to shred the "meat". Set aside.
Heat the vegetable oil in a pan over low to medium heat and add the onion to sauté for 2 minutes. Add the garam masala, coriander, turmeric, cumin, cardamom, paprika, nutmeg, nutritional yeast and stir though the onions before cooking for another minute.
Add the garlic and ginger and sauté for a minute before stirring in the shredded mushrooms and chickpeas.
Add the tomato passata, tomato paste, coconut milk, stock and water and stir well. Add the sea salt and increase the heat to high to bring the mixture up to a boil before reducing the heat to low again and cooking at a simmer for around 40 - 50 minutes or until the mixture has thickened.
Remove from the heat and stir through the baby spinach and vegan butter. Allowing the curry to sit for 5 minutes for the spinach to wilt. Serve with basmati rice and either papadums or naan bread.