Preheat Oven 450F. Steam the potatoes until tender.
Meanwhile in a large skillet saute onions, tomato paste and garlic in 1 tsp oil (or butter) and salt until soft and the rawness of the tomato paste goes away. Add the carrots and cook for another 1 minute. Add the mushrooms and the seasoning next until mushrooms gets soft.
Add the Seitan and chickpeas to the mixture and cook for 1-2 minutes.
Now add the vegetable broth to the vegetables. Mash the chickpeas a bit for the mixture to thicken. You can mix 1 tbsp all purpose flour and little water to make a thick paste and cook along with the mixture for 5 minutes. It will help it to thicken up. For me usually the chickpeas helps to bind the mixture.
Add the peas last (its frozen hence does not need to really cook for long). Toss it until combined. Taste and add more seasoning if required.
Take the potatoes hot and mash them to a smooth consistency. You can peel them if you want but I like the skin on my potatoes and keep them. Add in the butter and heavy cream. Season with salt and pepper to taste.
Place the vegetable mixture on a casserole dish or a deep pie pan. Then drop spoonfuls of the mashed potatoes around he perimeter of the dish.
Use a rubber spatula to spread the potatoes making sure to attach them to the rim of the pie plate. Seal the edges to prevent the filling from bubbling out of the plate in the oven. Place this dish on a baking sheet lined with foil just in case to catch any leeks.
Bake for 30-45 minutes until the edges are bubbling and the top is lightly browned. Cool for few minutes before serving.