First make the mushroom a la crème.
Melt the butter in a pan until it foams. Add the chopped onions, cover and sweat on a gentle heat for 5 - 10 minutes or until quite soft but not coloured. Meanwhile cook the sliced mushrooms in a little butter in a hot frying pan in batches if necessary. Season with salt and freshly ground black pepper and a tiny squeeze of lemon juice. Add the mushrooms to the onions in the saucepan, then add the cream and allow to bubble for a few minutes. Taste to check the seasoning and add the parsley and chives.
To make the potato mash.
Scrub the potatoes well, put them in a saucepan of cold water, add a pinch of salt and bring to the boil. When the potatoes are half cooked strain off two thirds of the water, replace the lid on the saucepan, put on a gentle head and allow them to steam until they are cooked. Peel immediately by just pulling off the skins, so you have as little waste as possible, mash while hot. Bring 300ml of milk to the boil. Add the egg into the hot mashed potatoes, and enough boiling creamy milk to mix to a soft light consistency. Add the freshly chopped herbs, butter and seasoning to taste.
Add the turkey to the mushrooms and pour the mix into a pie dish. Spread or pipe a layer of the potato on top. Reheat in a preheated oven 180C/350F/Gas 4 for 15 - 20 minutes until hot and bubbling. Serve immediately.