Shepherd's Pie (using Lamb or Mutton) is an economical meal,  favored in the Britain, Ireland and Australia, it can also be made using Beef, this is known as Cottage Pie.
Using chopped meat  mixed with gravy, with mashed potatoes, on top and baked until the top is crusty.
Cut 2 lbs of potatoes in 1" cubes.
Fill a pot with water, enough to cover the potatoes.
Bring to a boil, add the potatoes.
Bring to a boil again, reduce heat to simmer until potatoes are tender.
Cut 1 carrot, 1 onion and 1 celery stalk small and put in a bowl.
Place cooked potatoes in a bowl, hold back 1/2 cup if the liquid.
Mash potatoes with 1 tbls butter and the liquid, with salt and pepper, your choice,  to taste.
Heat 4 tbls vegetable oil in pan, add the onion, carrot, celery mixture and stir to coat vegetables.
Cook at medium, stir occasionally, until tender.
Add  1 1/2 lbs of ground meat and cook until meat is brown, not pink.
Do not leave meat in one clump, break it up as you cook.
Preheat oven to 400°F.
Remove excess fat, add 1 tbls flour and stir, cook for a couple of minutes.
Add 1 cup beef stock, 1 tsp dried rosemary and 1 tsp thyme.
Cook for  5 minutes, stirring until liquid thickens.
Add salt and pepper to taste.
Place meat  mixture in casserole dish, cover with mashed potatoes, make some patterns in the mashed potatoes to form spikes, this will add to crispy crust.
Put dish in middle rack in oven until crust forms, about 1/2 hour.
Let settle, for about 5-10 minutes, before serving

Place the shepherd's pie into the oven onto a rack in the center of the oven. After thirty minutes, the potatoes should have formed the golden brown crust. Remove from the oven and allow to rest ten minutes before serving.