Tonight, I needed to use up a packet of frozen diced turkey (which was an Asda "whoops" at £1 for the packet).
I fried the turkey in a little oil, then added chopped carrot, leeks and celery, and a little water - popped the lid on and cooked it for 10 mins. I then took the lid off and continued to cook until the liquid had reduced.
I then made a white sauce using the roux method, and added some dried Tarragon. I made the sauce fairly thick as when you add it to the turkey and veg, the liquid already in here makes the sauce a bit thinner. The whole lot got popped into an oven proof dish.
The rosti potato top is made by part cooking some jacket potatoes - don't cook fully. Then grate these onto the top of the pie. I sprayed with a little oil (or you can use butter) and cooked it in the oven (at 200 C) until it was browning nicely on top.