In a large, non-metallic bowl, mix the diced chicken, white wine, garlic and thyme. Cover and leave in the fridge for 1 hour to marinade.
Melt the butter and olive oil together in a large frying pan then add the onions and fry until softened. Add the water to the pan and bring to the boil. Add the chicken and its marinade to the pan then cook gently for 10 minutes.
Preheat the oven to Gas 5, 190°C, fan 170°C.
Next add the spinach and mustard to the pan and season with the salt and coarse white pepper. Once it has come back to the boil, reduce the heat and simmer for 5 minutes.
While the spinach is cooking, mix the cornflour with a little water in a small bowl water to form a thin, smooth paste. Add this to the pan and stir until the sauce thickens.
Add the grated Gruyère and crème fraîche and allow the cheese to melt into the sauce then remove the pan from the heat and cool for 10 minutes.
Roll out the shortcrust pastry and line a flan tin with it.
Spoon the cooled filling mixture into the shortcrust case and roll out the puff pastry. Cut it into thin strips and make a lattice over the top of the pie. You may need to use a little water to stick the puff pastry strips to the base. Trim the edges and glaze with beaten egg. Place back into the oven and bake for 20 to 25 minutes, until the puff pastry is risen and golden.
Serve with salad, new potatoes and a glass of white wine.
You can also use single or double cream if you don't mind the extra calories.
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