Special equipment: five 5-inch cast-iron skillets or a 10-inch cast-iron skillet
Preheat the oven to 400 degrees F.
Melt the butter in a large cast-iron skillet set over medium heat. Add the carrots, celery and onions, and saute until tender, 3 to 5 minutes. Add the ground beef and cook until no longer pink, 5 to 7 minutes. If using ground beef substitute, cook until warmed through, about 2 minutes. Sprinkle the flour over the top and saute to cook off the flour flavor, 1 minute. Whisk in the porter and stock until combined. Stir in the corn, peas, Worcestershire, garlic powder and red pepper flakes until combined. Bring the mixture to a simmer and cook to reduce the liquid by one-third, 3 to 5 minutes. Remove from the heat and season with salt and pepper.
Fill five 5-inch cast-iron skillets halfway with the beef mixture. Divide the Pepper Jack Mashed Potatoes evenly among the skillets. Alternatively use a 10-inch skillet and fill halfway with the beef mixture and top with the potatoes. Place the skillets on a baking sheet and bake until the mashed potatoes start to brown and the sauce is bubbly, 20 to 30 minutes.
Place the potatoes in a large saucepan set over medium-high heat, then cover with cold water and season with salt. Bring to a boil and cook until the potatoes are tender, 10 minutes. Drain and place the potatoes in the bowl of a stand mixer fitted with the paddle attachment. Add the sour cream, cream cheese, pepper Jack and some salt and pepper, and mix until fluffy. Taste and adjust the seasoning as needed. Fold in the green onions and enjoy!
If you want this to be a vegetarian meal, use vegetarian Worcestershire sauce.