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, for which you have a lid. Arrange the mushrooms so they are domed side up, then top with a piece of parchment paper, pushing it down to cover all the ingredients. Cover with the lid, then transfer to the oven for 1 hour. Turn the mushrooms over, replacing the paper and lid, and return to the oven for 20 minutes more, or until the mushrooms are very tender but not falling apart. Use a pair of tongs to transfer the mushrooms to a chopping board, then cut them in half and set aside.
Use a spoon to remove the onion, garlic and chilli (discarding the stem) – don’t worry if you scoop up some of the spices and oil. Put them into the small bowl of a food processor and blitz until smooth. Return the blitzed onion mixture to the saucepan, along with the mushroom halves, and place on a medium-high heat. Cook for about 5 minutes, for all the flavours to come together.
While the mushrooms are cooking, make the mash by putting the beans into a food processor along with the lemon juice, olive oil, ½ teaspoon flaked salt and 2 tablespoons of water. Blitz until completely smooth. Transfer to a medium saucepan and cook on a medium-high heat for about 3 minutes, stirring, until warmed through.
To serve, divide the butter bean mash between four plates. Top with four mushroom halves per plate and spoon over a generous amount of the oil and its accompanying aromatics (you won’t need all of it, though – see intro).