Comes highly recommended. Takes at least an hour.
Heat the oil, softly let the onion rings simmer for a long time, in the meantime chop the leeks and garlic.
/*Heat 2 tablespoons oil in a 5-quart heavy pot over medium-high heat until it shimmers. Brown seitan in 3 batches, turning occasionally, 3 to 5 minutes (add 2 tablespoons oil between batches). Transfer to a plate.*/
Cook leeks until softened, about 6 minutes. Add garlic and cook, stirring frequently, until golden and fragrant, about 2 minutes.
Add mushrooms and 1/2 teaspoon salt and cook, covered, stirring occasionally, until they just begin to give off liquid, about 5 minutes. Add carrots, parsnips, thyme, and rosemary and cook, covered, stirring occasionally, until vegetables are just tender, 10 to 12 minutes. Transfer vegetables to a bowl.
Add (uh it’s empty now right) wine to pot and boil until reduced to about 1 cup, 10 to 15 minutes.
While wine reduces, make a beurre manié by stirring together butter and flour in a small bowl to form a paste.
Add stock to wine and bring to a brisk simmer. Whisk in beurre manié, then simmer, whisking occasionally, until thickened slightly, 3 to 5 minutes. Add seitan, pearl onions, and vegetable mixture to pot and simmer, covered, 30 minutes.
Remove from heat and stir in parsley.
Make topping while stew simmers:
Peel (I washed) potatoes and cut into 2-inch pieces. Cover potatoes and celery root with cold water in a 4-to 5-quart heavy pot, then simmer, partially covered, until very tender, 20 to 25 minutes. Reserve 1/2 cup cooking water, then drain in a colander.
Bring butter, milk, and cream to a simmer in pot over medium heat, stirring until butter has melted. Stir in reserved cooking water, 1 teaspoon salt, and 1/2 teaspoon pepper. Remove from heat. Force potatoes and celery root through ricer into hot milk mixture and stir gently to combine. (I just drained the potatoes, added spices and some milk, bob’s your uncle.)
Finish pie: I skipped this.
Preheat broiler. Put baking dish in a large 4-sided sheet pan and transfer stew to dish.
Spoon potato mixture over stew and spread evenly to cover. Broil about 3 inches from heat until top is golden, about 5 minutes.