melt the butter and add the chopped onion. Season with salt and freshly ground black pepper and cook
without browning
on a gentle heat for 8-10 minutes
until the onions are completely soft.
Add the cream and stock and bring up to the boil and simmer for a few minutes to thicken.
Meanwhile
in a separate frying pan
heat the mushrooms with a knob of butter (you might need to do this in two batches) for 3-4 minutes
or until soft and golden brown.
Add tarragon or majoram to the mushrooms and season to taste with salt and freshly ground black pepper.
Add the meat and the mushrooms and stir well.
Meanwhile for the mashed potato
place the potatoes in a saucepan of boiling water and cook until soft. Drain the potatoes and return them to the saucepan. Mash roughly and then add the butter
egg yolk and milk and mash to a thick paste. Season
to taste
with salt and freshly ground black pepper.
Pour the cream
mushroom and meat mixture into a shallow gratin dish and top with the mashed potato.
Place in the oven for about 20-30 minutes until the potato topping is golden brown and the filling is hot.
