Preheat the oven to 400 degrees F. Make the potatoes: Put the potatoes in a large saucepan and cover with cold salted water. Bring to a boil. Cook until the potatoes are tender, 12 to 14 minutes. Reserve 1 cup of the cooking water, then drain and place the potatoes in a stand mixer fitted with the whisk attachment.
Combine the sour cream and egg, then add to the potatoes; season with a pinch of salt and 1/2 teaspoon pepper. Whip, adding 1/4 cup of the reserved cooking water at a time, until smooth and fluffy, 2 to 3 minutes. Taste and adjust the seasoning as needed. Fold in all but 1 tablespoon of the scallions and set aside.
Make the filling: Melt the butter in a large cast-iron skillet over medium heat. Add the carrots, celery, onion and beef substitute and saute until tender, 4 to 6 minutes. Sprinkle the flour over the top and saute to cook off the flour flavor, about 1 minute. Whisk in the beer and stock until combined. Stir in the corn, edamame, steak sauce, garlic powder and red pepper flakes until combined. Bring the mixture to a simmer and cook to reduce the liquid by one-third, 2 to 4 minutes. Remove from the heat and season with salt and pepper.
Fill six 4-inch cast-iron skillets (or ramekins) halfway with the filling. Divide the mashed potatoes among the pans. Place the skillets on a baking sheet and bake until the mashed potatoes start to brown and the sauce is bubbly, 20 to 25 minutes. Top with the remaining scallions.
For a meat version, replace the ground beef substitute with 1 pound lean ground beef; saute 5 to 7 minutes in step 3 to cook through.