Trim fat and gristle from leftover meat and cut into small cubes.
Melt butter in saucepan and saute onion, garlic and celery over low heat for 10 minutes.
Stir in flour and cook until it is light gold in colour.
Blend in gravy and stock and cook until bubbling.
Add meat and parsley and remove from heat.
Season to taste with salt and pepper.
Add 1 cup of frozen peas or corn (optional)
Line sides of deep, oven-proof dish with layer of mashed potatoes, add meat mixture, spread remaining potatoes on top. Smooth with a knife and decorate with a fork.
Cook in 350-375 degree oven for 30 minutes or until potato top is browned.