Ingredients

• 3 pounds russet potatoes, unpeeled
• 3 pounds Yukon Gold potatoes, unpeeled
• 1/2 cup (1 stick) unsalted butter, cut into 1/2" cubes
• 1 1/2-2 cups whole milk, warmed
• Kosher salt
• 1 ounce dried porcini mushrooms
• 3/4 cup brown or French green lentils
• 6 garlic cloves, divided, plus 2 tablespoons chopped garlic
• 1 teaspoon kosher salt plus more
• 5 tablespoons olive oil, divided
• 3 cups coarsely chopped onions
• 2 tablespoons tomato paste
• 2 bay leaves
• 2 cups dry white wine
• 8 cups vegetable broth
• 2 tablespoons cornstarch
• 2 tablespoons gluten-free white miso or 2 teaspoons gluten-free tamari soy sauce
• Freshly ground black pepper
• 12 cups 1/2" pieces peeled fall vegetables (such as squash, turnips, carrots, and parsnips)
• 1 cup frozen pearl onions, thawed, halved
• 2 4" rosemary sprigs
• 2 cups bite-size pieces mixed fresh mushrooms
• 1/4 cup chopped mixed fresh herb (such as parsley, chives, and sage)

Instructions

1. Preheat oven to 450°F. Bake potatoes on a foil-lined baking sheet until tender, about 45 minutes. Let cool slightly, then peel. Press potatoes through a ricer, food mill, or colander into a large bowl. Add butter; stir until well blended. Stir in milk. Season to taste with salt. DO AHEAD: Potatoes can be made 1 day ahead. Let cool, press plastic wrap directly onto potatoes, and chill.
2. For filling: Soak dried porcini in 3 cups hot water; set aside. Combine lentils, 1 garlic clove, 1 teaspoon salt, and 4 cups water in a medium saucepan. Bring to a boil; reduce heat and simmer, stirring occasionally, until lentils are tender but not mushy, 15ñ20 minutes. Drain lentils and discard garlic. Heat 3 tablespoons oil in a large heavy pot over medium heat. Add onions and cook, stirring occasionally, until soft, about 12 minutes. Add chopped garlic and cook for 1 minute. Stir in tomato paste. Cook, stirring constantly, until tomato paste is caramelized, 2-3 minutes. Add bay leaves and wine; stir, scraping up any browned bits. Stir in porcini, slowly pouring porcini soaking liquid into pan but leaving any sediment behind. Bring to a simmer and cook until liquid is reduced by half, about 10 minutes. Stir in broth and cook, stirring occasionally, until reduced by half, about 45 minutes. Strain mixture into a large saucepan and bring to a boil; discard solids in strainer. Stir cornstarch and 2 tablespoons water in a small bowl to dissolve. Add cornstarch mixture; simmer until thickened, about 5 minutes. Whisk in miso. Season sauce with salt and pepper. Set aside. Preheat oven to 450°F. Toss vegetables and pearl onions with remaining 2 tablespoons oil, 5 garlic cloves, and rosemary sprigs in a large bowl; season with salt and pepper. Divide between 2 rimmed baking sheets. Roast, stirring once, until tender, 20-25 minutes. Transfer garlic cloves to a small bowl; mash well with a fork and stir into sauce. Discard rosemary. DO AHEAD: Lentils, sauce, and vegetables can be made 1 day ahead. Cover separately; chill. Arrange lentils in an even layer in a 3-quart baking dish; set dish on a foil-lined rimmed baking sheet. Toss roasted vegetables with fresh mushrooms and chopped herbs; layer on top of lentils. Pour sauce over vegetables. Spoon potato mixture evenly over. Bake until browned and bubbly, about 30 minutes. Let stand for 15 minutes before serving.
3. Soak dried porcini in 3 cups hot water; set aside. Combine lentils, 1 garlic clove, 1 teaspoon salt, and 4 cups water in a medium saucepan. Bring to a boil; reduce heat and simmer, stirring occasionally, until lentils are tender but not mushy, 15ñ20 minutes. Drain lentils and discard garlic.
4. Heat 3 tablespoons oil in a large heavy pot over medium heat. Add onions and cook, stirring occasionally, until soft, about 12 minutes. Add chopped garlic and cook for 1 minute. Stir in tomato paste. Cook, stirring constantly, until tomato paste is caramelized, 2-3 minutes.
5. Add bay leaves and wine; stir, scraping up any browned bits. Stir in porcini, slowly pouring porcini soaking liquid into pan but leaving any sediment behind. Bring to a simmer and cook until liquid is reduced by half, about 10 minutes. Stir in broth and cook, stirring occasionally, until reduced by half, about 45 minutes.
6. Strain mixture into a large saucepan and bring to a boil; discard solids in strainer. Stir cornstarch and 2 tablespoons water in a small bowl to dissolve. Add cornstarch mixture; simmer until thickened, about 5 minutes. Whisk in miso. Season sauce with salt and pepper. Set aside.
7. Preheat oven to 450°F. Toss vegetables and pearl onions with remaining 2 tablespoons oil, 5 garlic cloves, and rosemary sprigs in a large bowl; season with salt and pepper. Divide between 2 rimmed baking sheets. Roast, stirring once, until tender, 20-25 minutes. Transfer garlic cloves to a small bowl; mash well with a fork and stir into sauce. Discard rosemary. DO AHEAD: Lentils, sauce, and vegetables can be made 1 day ahead. Cover separately; chill.
8. Arrange lentils in an even layer in a 3-quart baking dish; set dish on a foil-lined rimmed baking sheet. Toss roasted vegetables with fresh mushrooms and chopped herbs; layer on top of lentils. Pour sauce over vegetables. Spoon potato mixture evenly over.
9. Bake until browned and bubbly, about 30 minutes. Let stand for 15 minutes before serving.