Thaw the puff pastry in advance of starting.
Peel and chop or crush the garlic. Peel and chop the onion; set both aside.
Slice or quarter the mushrooms, set aside.
Chop the chicken in smallish pieces, set aside.
Heat a few good glugs of oil in a large frying pan over medium-high heat.
Add the chicken, and stir fry it for about 5 minutes or until it's all browned. Remove from the frying pan, set aside.
Add a bit more oil to the frying pan, and toss in the onion, garlic and mushrooms. Toss them around for about 10 minutes, then crank the heat a bit and pour in the white wine and the stock.
Start heaving oven to 180 C / 350 F / Gas Mark 4.
Add the chicken back into the frying pan, and let it simmer energetically for about 10 minutes. Lower the heat, add the cream and the parsley, along with a few pinches or grinds of pepper, simmer for a final 5 minutes. It's okay if the sauce is somewhat liquidy, as it will thicken more in baking later. (Don't be tempted to thicken it; we did one time, and the pie ended up dry with no sauce.)
Pour into the ovenproof dish. Roll out the puff pastry to fit the top of your dish, and cover the dish with the pastry dough. Brush with milk or butter quickly melted in the microwave, then pop into the oven for about 30 to 40 minutes or until the crust is cooked and a lovely golden brown, and the dish is bubbling. Serve hot.