Cut the chicken breasts into small pieces – I like mine about 1 to 2 inches square.
Heat 2 tablespoons of the olive oil in a large pan until very hot. Brown the chicken pieces a few at a time, depending on the size of your pan. When they’re all done, put them to one side.
Add the rest of the oil to the pan. Add the onion and leek, cover and turn the heat down low. Cook like this for about 10 minutes until the onion and leek are nicely softened. Then add the flour, stir in well and cook until completely incorporated, which will take a couple of minutes or so.
Add a cupful of the stock and stir. The sauce should immediately start to thicken. Add the rest of the stock, bit-by-bit, until you have about a third left.
Add the chicken pieces back to the pan. Quarter the mushrooms and add them too. If the sauce seems a bit too thick, add a bit more stock. Add the thyme.
Cook on a medium heat, uncovered, for about 10 minutes, and then pour in the cream. Cook for a further 10 minutes.
If you are adding the potatoes, boil them separately and then chop into smallish pieces. Add to the sauce.
Cook until the sauce has reduced to your preferred consistency. If it’s too thick, add a bit more stock to thin it out. Season with salt and pepper to taste.
Pour the sauce into a shallow ovenproof dish and top with the puff pastry. Cook at 200°C, Gas Mark 6 for 20-30 minutes, until the pastry is golden brown.
Serve with veggie of your choice. Buttered carrots with parsley would go nicely, as would green beans.