Thinly slice the mushrooms, onion and garlic.
Heat the oil in a large frying pan. Add the onion and gently sauté for a few minutes.
Add the mushrooms and garlic and stir for a minute. Add the chilli flakes, cover the pan with a lid, and cook for 8 minutes over low heat.
Add the vinegar, let it sizzle, then pour over the stock. Add the kale, put the lid on again and cook until the mushroom and kale have softened. This should take about 5-8 minutes.
Add the beans, turn up the heat and cook for 3 minutes to reduce the sauce.
Serve with some extra chilli flakes, if you like.