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Preheat the oven to 150°C fan.
Put all the ingredients for the steaks and 1 tablespoon of flaked salt into a large ovenproof saucepan. Cover and bake for 1 hour. Turn mushrooms over, cover again, and bake for 20 more minutes.
Remove onion, garlic, and chilli and blend into a smooth mixture. Return to the saucepan with the mushroom halves and cook for 5 minutes.
Make the butter bean mash by blending beans, lemon juice, olive oil, salt, and water until smooth. Heat through in a saucepan.
Serve by dividing the mash between plates, topping with mushroom halves, and spooning over oil and aromatics.