Exact quantities are not too important with this.
I would use:
3 or 4 cups of diced cooked lamb.
1 med onion diced
any leftover cooked vegetables cut up small
1 cup mushrooms sliced
Enough leftover gravy to mix in with it all to make it moist.
Place it all in a greased oven dish.
Boil about 6 large potatoes until cooked and then mash with butter and milk. Spread potato
over the meat mixture and fork across the top of it. Season the meat mixture with salt and pepper if you wish.
Bake at 350 for approx 45mins, for the last 10mins sprinkle some grated cheese over the top and let it brown.