Bring 1 c stock to boil, add seitan or meatballs, turn off heat, and let sit for 10 minutes. Drain and reserve the stock.
Meanwhile, heat 2 T of oil in frying pan and sauté onions for 3 minutes. Transfer to a bowl.
Heat remaining oil, and sauté mushrooms for 3 minutes. Add onions and “meat” and cook for 1 minute. Slowly stir in flour, then add all the stock. Stir frequently until thickened, 3-5 minutes.
Meanwhile, cook and drain the pasta.
Add milk, mustard, salt, and pepper to the sauce and simmer, stirring frequently, for 10 minutes. Remove from heat, stir in sour cream, and serve over pasta.