- 10 ounces pearl onions (about 2 1/4 cups)
- 9 tablespoon olive oil, divided
- 1 1/2 pounds seitan, sliced 1/4 inch thick and patted dry
- 3 medium leeks (white and pale green parts only), halved lengthwise, sliced 1/2 inch thick, and washed
- 3 garlic cloves, chopped
- 1 pound cremini mushrooms, trimmed and quartered
- 3/4 pound carrots (about 4 medium), cut into 3/4-inch-thick pieces
- 3/4 pound parsnips (about 4 medium), cut into 3/4-inch-thick pieces
- 2 tablespoons thyme leaves
- 1 tablespoon chopped rosemary
- 1 (750-ml) bottle full-bodied red wine such as Burgundy or Côtes du Rhône
- 3 tablespoons unsalted butter, softened
- 3 tablespoons all-purpose flour
- 3 1/2 cups hot rich vegetable stock
- 1/2 cup chopped flat-leaf parsley
- 2 1/2 pounds Yukon Gold potatoes
- 3/4 pound celery root, peeled and cut into 1-inch pieces
- 1/2 stick unsalted butter
- 1/2 cup whole milk
- 1/2 cup heavy cream
- Equipment: a 3-quart shallow (2-inch-deep) flameproof baking dish; a potato ricer or a food mill fitted with medium disk
Blanch pearl onions in a 2-quart saucepan of boiling water 2 minutes, then drain and cool in an ice bath to stop cooking. Peel onions and trim, leaving root ends intact.
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 in remaining 3 tablespoons oil with 1/4 teaspoon salt in pot over medium heat, covered, stirring occasionally, 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 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 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.
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.