Heat 2 tablespoons olive oil and soy sauce in a skillet over medium-high heat. Saute seitan in hot oil until browned on all sides, about 5 minutes.

Heat remaining olive oil in a large pot over medium-high heat. Saute onion, celery, carrots, potatoes, and garlic in hot oil until onions are soft, 3 to 5 minutes. Reduce heat to medium and slowly stir beer into vegetable mixture.

Stir brown sugar, flour, thyme, salt, and black pepper into beer mixture; add seitan. Bring mixture to a simmer, reduce heat to low, and cook until stew reduces and thickens, about 45 minutes.