Melt the butter over medium heat in a large pot. Once the butter has melted, add the onion, carrots, and celery. Cook the vegetables for about 5 minutes, stirring occasionally, until they start to soften.
Stir in the seitan and cook it with the vegetables for about 5 minutes, stirring occasionally, until the pieces just start to brown around the edges.
Sprinkle the flour into the pot and add the garlic. Stir well until the flour evenly coats the seitan and vegetables. Cook the mixture while stirring constantly for 2 minutes.
Slowly add the milk to the pot while stirring, then add the broth, nutritional yeast, and poultry seasoning. Stir well to break up any clumps of flour. Raise the heat to high and bring the stew to a boil, then reduce the heat and let it simmer for about 15 minutes, until the vegetables are tender.
While the stew simmers, preheat the oven to 400°F.
To make the biscuit topping, first stir the milk and apple cider vinegar together in a small bowl or container.
Whisk the flour, baking powder, and salt together in a large mixing bowl. Cut the butter into 5 or 6 slices, then add them to the flour mixture. Use a fork or pastry cutter to cut the butter into the flour mixture. Continue until the mixture resembles coarse crumbs.
Pour the milk mixture into the bowl with the flour mixture and stir just until fully mixed. You should have a fluffy dough at this point.
Once the stew has finished cooking, transfer it to an approximately 2 ½ quart baking dish. Scoop the biscuit dough into 8 balls and distribute them over the stew.
Bake the casserole for about 25 minutes, or until the stew is bubbly and the biscuits are cooked through and lightly browned on top. I recommend using a spoon to raise one of the biscuits to make sure the bottom is cooked. It may be moist and covered with stew, but you should not see any raw dough.
Let the casserole sit for 5 to 10 minutes before serving.