Cook the pasta according to package directions, reserve one cup of cooking water. Drain the pasta in a colander, set aside.
Add the onion, garlic, carrots, celery, and seitan to a food processor and process until it resembles a ground meat consistency.
Heat a large non-stick skillet over medium heat. Add the vegetable and seitan mixture to the skillet and saute until the vegetables are tender and seitan has started to dry out slightly, about 3-6 minutes. Add the tomato paste and stir to combine, let the paste caramelize slightly, cooking it for 3 minutes, until it darkens in color. Add the tamari and mix to combine. Add the pumpkin, cooking water, milk, vinegar, oregano, and salt, mix to combine. Allow to simmer for a few minutes until heated through.
Add the pasta to the sauce and toss to coat the pasta.
Serve topped with vegan parmesan and chopped parsley, if desired