In a medium bowl, mix together arrowroot and tamari. Add stock, wine, water and tomatoes. Set aside.
Heat canola oil in a large skillet, add onion, celery, garlic and carrots and sauté on medium heat for 10 minutes. Add mushrooms and bell pepper, and sauté 5 minutes more. Add seitan, along with the sauce mixture, herbs and a generous amount of black pepper. Simmer 5 minutes, until sauce is nicely thickened. (If it’s too thick, add more water; if it’s too thin, mix some more arrowroot with water and stir in.) Add peas and walnuts.
To make mashed potatoes for the topping: Place potatoes in cold water to cover by 1 inch. Bring to a boil, reduce heat and simmer until tender, about 15 minutes. Drain in colander. Return potatoes to pan and shake over high heat for a minute to remove excess moisture. Add the olive oil, soy milk, mustard and salt and pepper to taste, and mash with a potato masher until smooth.
Place the seitan mixture in a shallow ceramic baking dish. Spread mashed potatoes over the top, smoothing with a spoon. Sprinkle with paprika.
Bake uncovered at 350 degrees, for 40 minutes or until bubbly and starting to get a bit crusty on top.