Makes:4 to 6 servings
Shepherd’s pie is known for its rich meat gravy and blanket of mashed potatoes. This vegetarian version delivers an equally hearty filling of root vegetables in savory sauce covered with a thick layer of mashed russet and sweet potatoes.
2 medium russet potatoes, peeled and cut into large dice
1 medium sweet potato, peeled and cut into large dice
3/4 cup plain yogurt
1 teaspoon kosher salt
1/4 teaspoon freshly ground black pepper
2 tablespoons olive oil
10 ounces cremini mushrooms, cleaned, stemmed, and quartered
Heat the broiler to low and arrange a rack in the upper third of the oven. Line a baking sheet with aluminum foil; set aside.
Place russet and sweet potatoes in a large pot of heavily salted water and bring to a boil over high heat. Simmer until just tender when pierced with a knife, about 5 minutes. Drain, then return potatoes to the pot. Mash coarsely; add yogurt, salt, and pepper; and stir until just combined. Set aside.
Heat olive oil in a large frying pan over medium heat until shimmering. Add mushrooms and cook, stirring rarely, until browned, about 6 minutes. Add onion, season with salt and freshly ground black pepper, and cook, stirring occasionally, until onion just begins to soften, about 3 minutes.
Add carrots, celery root, tomato paste, and thyme and stir to coat vegetables. Add mushroom broth and soy sauce and stir until incorporated. Bring to a simmer, stirring occasionally, and cook until vegetables are just tender when pierced with a knife, about 10 minutes.
Meanwhile, combine water and flour in a small bowl and stir until smooth. When vegetables are ready, add flour mixture, stir to combine, bring to a boil, and cook until sauce has thickened, about 1 minute. Season to taste with salt and freshly ground black pepper. Stir in frozen peas.
Transfer mixture to a 7-by-11-inch baking dish (or a 3-quart baking dish) and spread reserved mashed potato mixture over top. Place on the prepared baking sheet and broil until the top starts to brown, about 5 minutes. Let cool 5 to 10 minutes before serving.