In a small bowl, whisk together the stout, agave, tomato paste, mustard along, ½ teaspoon salt and ½ pepper. Set aside.
Preheat oven to 375°.
Place the cabbage and carrots in a large casserole dish. Drizzle a tablespoon of oil over the vegetables, sprinkle with the rest of the salt and pepper and then toss gently. Bake uncovered for 25 – 30 minutes, stirring after about 15 minutes.
While the vegetables are cooking, heat the remaining olive oil in a large pan over medium high heat. Add the onion and cook for about 5 minutes, stirring frequently, until it begins to brown
Add the seitan to the pan and cook for about 10 minutes, or until it begins to brown. Pour the stout mixture over the onions, and sprinkle with the flour. Stir everything together, and cook for about 3 more minutes, until the mixture is bubbly.
Once the carrots and cabbage mixture is done roasting, remove from oven and spoon the onion and seitan mixture over it.
Garnish with parsley and serve hot.