Cut up your leftover smoked brisket into small pieces, this is one thing I would change about how I made mine is that I cut the brisket into smaller pieces.
Startup a pot of water to boil for your potatoes.
Peel and cut your potatoes, I used russet potatoes but you can use a different kind if you want.
Boil your potatoes until they are ready to be mashed.
In a mixer mash your potatoes, and combine with your potatoes the milk and sour cream, add in salt and pepper them as well until they are well mixed and blended.
Chop your onion and peppers up into small pieces.
Oil your cast iron pan, I used a large skillet to make mine in if you use a smaller skillet you will want to use less meat and potatoes to make this with.
In a mixing bowl combine the Cream of Mushroom soup, peppers, onion, and minced garlic together and mix well.
Mix in the chopped-up brisket to the mixing bowl with the soup mixture.
Spread this mixture into the bottom of the skillet.
Spread the mashed potatoes onto the soup mixture in the bottom of the skillet.
Start your smoker rolling, I had mine set at a setting of P4 then turned it up to 350 after it was good to go.
Allow to smoke in your smoker for about 30 minutes, I did turn my skillet a couple of times while it was smoking.
Top with cheese after 30 minutes and then top with cheese.
Allow the shepherd's pie to finish smoking until the cheese has fully melted.
Remove and serve!