Heat the oven to 220°C/fan200°C/gas 7.
In a large deep frying pan, heat the butter and olive oil and add the onions, sliced leeks and carrots. Gently fry for about 10 minutes until starting to soften.
Add the chicken and cook, stirring, for another 4-5 minutes. Stir in the garlic, add the stock and bubble away until reduced by half. Add the cream and dried thyme, bring to the boil, season then simmer for 5-6 minutes until thickened.
Spoon the pie filling into a 2.5-litre pie dish then mix in the spinach. Set aside to cool.
Unroll ready-rolled puff pastry and drape it over the dish. Trim any excess, press down on the edges with a fork to seal, then cut a small hole in the centre. Cut shapes from the trimmings to decorate the top. Beat the egg and brush over the pastry, then bake for 25-30 minutes until golden.
If the pie filling seems too wet, sprinkle a little cornflour into the sauce and mix well until you have the desired consistency.