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Heat half the oil in a large frying pan. Add the onion and cook for 5 mins until softened, then push to one side. Squeeze the meat from the sausages and roll into balls. Add to the pan and fry for 5 mins until golden. Add the mushrooms and cook for 5 mins more until softened.
Season, then stir in the mustard, crème fraîche and 4 tbsp water. Bubble for 2 mins, then remove from the heat and scatter in the parsley.
Heat oven to 200C/180C fan/gas 6. Transfer sausage mixture to 2 small pie dishes or 1 large baking dish. Toss the ciabatta in the remaining oil, scatter over the top of the pies and bake for 20 mins until golden and bubbling.