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Heat 2 tbsp of the oil in a saucepan or large flameproof casserole dish. Fry the onions for 10 mins until soft, then add the mushrooms and bacon and fry for another 5 mins until golden. Stir in the flour and cook for 2 mins more. Gradually pour in the stock, then bring to the boil and bubble for 2 mins. Add the kale and cook for another 5 mins, then stir in the mustard, crème fraîche, half the tarragon and the lemon juice. Spoon the mushroom mixture into four small baking dishes.
Heat the oven to 180C/160C fan/gas 4. Put the butter beans in a food processor with the remaining oil and tarragon and the lemon zest. Blitz until chunky, adding 2-3 tbsp water to loosen if needed.
Spoon the butter bean mixture over the filling, smoothing with the back of a spoon. Bake for 20-25 mins until golden. Leave to cool slightly, then serve.