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Heat oven to 200C/180C fan/gas 6. Heat the oil in a large, shallow casserole dish on a medium heat. Add the spring onions and fry for 3 mins, then stir through the frozen spinach and cook for 2 mins or until it’s starting to wilt. Remove the skin from the chicken and discard. Shred the chicken off the bone and into the pan, and discard the bones. Stir through the stock and mustard. Bring to a simmer and cook, uncovered, for 5-10 mins.
Stir in the peas, crème fraîche and herbs, then remove from the heat. Scrunch the filo pastry sheets over the mixture, brush with a little oil and bake for 15-20 mins or until golden brown.