Easy Shepherd's Pie

Instructions

1. 1Heat the broiler to low and arrange a rack in the upper third of the oven. Line a baking sheet with aluminum foil; set aside.
2. 2Place 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.
3. 3Heat 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.
4. 4Add 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.
5. 5Meanwhile, 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.
6. 6Transfer 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.