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Heat ½ tbsp oil over a gentle heat in a flameproof casserole dish. Add the onion and cook for 5 mins until softening. Scatter over the thyme and garlic, and stir for 1 min. Turn up the heat and add the chicken, frying until golden but not fully cooked. Add the mushrooms and the remaining oil. Heat oven to 200C/180 fan/gas 6.
Add the stock, crème fraîche, mustard and kale, and season well. Add the cornflour mixture and stir until thickened a little.
Remove from the heat and cover with the puff pastry lid, pressing into the sides of the casserole dish. Slice a cross in the centre and glaze with the egg. Bake for 30 mins until the pastry is puffed up and golden.