Peel your potatoes and chop them into cubes about 1-½” thick.
Add the potatoes to a pot of boiling, salted water, and let simmer for 20 minutes or until the potatoes are fork tender.
Strain the potatoes, then add them to a mixing bowl with the cheddar cheese, cream cheese, heavy cream, salted butter, salt, and pepper. Mash until smooth, then set aside to cool.
In an oven proof pan over medium heat, saute the shallot in olive oil until translucent, then add the garlic and cook for a minute more.
Add the brisket and Worcestershire sauce to the pan. Stir to combine. Let it cook for 5-7 minutes, stirring occasionally, until the brisket is heated through and starts to break down.
Sprinkle the flour on top, mix it in, then add the beef broth and give it a good stir. Bring to a simmer for 5-7 minutes, stirring occasionally, until it thickens into a gravy-like consistency.
Add the frozen veggies and mix until well incorporated.
Spread the cheddar mashed potatoes on top to fully cover the filling, then bake at 350°F for 30-40 minutes, or until the potatoes are golden brown and just starting to get crispy on top.
Serve immediately with (optional) green onions on top.