Heat oil in large skillet or saucepan. Add onion, garlic, and mushrooms. Sauté till onion is translucent and garlic is soft. Add carrots, celery, tamari, seitan and seasonings, stirring constantly for about 10 minutes. Slowly add soymilk and up to ¼ cup water, then thickener, dissolved in 1-tablespoon water. Cover and simmer for 10 to 15 minutes. Serve over rice, garnished with sprig of fresh parsley.