Preheat the oven to 180 C / Gas 4. Grease a 1 litre baking or casserole dish.
In a saucepan over medium heat, add the turkey, gravy, water, salt, pepper and savoury seasoning. Once simmering, add the chicken stock cube (if needed), along with the carrot, peas and onion. Bring to the boil, and cook till thickened (the mixture should not be too thick).
Pour the turkey mixture into the prepared 1 litre baking or casserole dish. Arrange sliced potatoes over the top, overlapping, to cover the entire pie. Brush the potatoes with melted butter and sprinkle with 1/2 teaspoon salt. Cover with aluminium foil or a lid.
Bake in the preheated oven for 30 minutes. Uncover for final 15 minutes so that the potatoes will brown. Let the pie rest for 15 minutes before serving.
If you can't get your turkey and gravy mixture thick enough, thicken with flour or with cornflour. Also, you can grill the top of the pie at the very end so that your potatoes will brown even more.