Begin by preheating your oven to 450°F (230°C). Place the potatoes on a foil-lined baking sheet and bake them until they are fork-tender, which should take about 45 minutes. Once out of the oven, let them cool slightly, then peel them carefully. To achieve a luxuriously smooth texture, press the potatoes through a ricer, food mill, or colander into a large mixing bowl. Add the butter to the mashed potatoes, stirring until fully melted and incorporated, offering a rich creaminess. Pour in the milk gradually and continue stirring to blend everything well. Season generously with salt to taste, setting the potato topping aside for now. Next, for the filling, soak the dried porcini mushrooms in 3 cups of hot water, setting them aside to rehydrate while you prepare the lentils. In a medium saucepan, combine the brown lentils, one garlic clove, 1 teaspoon of kosher salt, and 4 cups of water. Bring it to a boil, then reduce the heat and simmer, stirring occasionally, until the lentils are just tender but not mushy—this should take about 15 to 20 minutes. Drain the lentils and discard the garlic clove. In a large heavy pot, heat 3 tablespoons of olive oil over medium heat and add the chopped onions. Cook them, stirring occasionally, until soft and translucent—around 12 minutes. Add the remaining minced garlic and cook for an additional minute until fragrant. Stir in the tomato paste and cook, stirring constantly, until it caramelizes and darkens slightly, about 2 to 3 minutes. Add the bay leaves and pour in the white wine, scraping the bottom of the pot to lift any browned bits for added flavor. Introduce the soaked porcini mushrooms along with the soaking liquid, taking care to leave any sediment behind. Bring the mixture to a simmer and cook until the liquid reduces by half, approximately 10 minutes. Add the vegetable broth and continue to simmer, stirring occasionally, until the liquid again reduces by half, around 45 minutes. To create a silky sauce, strain this mixture into a large saucepan and bring it to a boil. In a small bowl, dissolve the cornstarch in 2 tablespoons of water and stir it into the boiling sauce. Simmer until thickened, about 5 minutes, then whisk in the miso paste for an added depth of flavor. Season with salt and freshly ground pepper to your liking and set the sauce aside. Increase your oven temperature to 450°F (230°C) again. In a large bowl, toss the chopped mixed vegetables along with pearl onions, the remaining 2 tablespoons of olive oil, five smashed garlic cloves, and rosemary sprigs. Season with salt and pepper generously. Spread the mixture evenly onto two rimmed baking sheets and roast for 20 to 25 minutes, stirring once halfway through, until the vegetables are tender and slightly caramelized. Once roasted, remove the garlic cloves from the baking sheets, mash them well with a fork, and stir this flavorful roasted garlic into your prepared sauce. Discard the rosemary sprigs. To assemble, arrange the cooked lentils in an even layer in a 3-quart baking dish set on a foil-lined rimmed baking sheet to catch any drips. Toss the roasted vegetables with the fresh mushrooms and chopped herbs such as parsley, sage, and chives, and layer this over the lentils. Pour the thickened miso sauce evenly over the vegetable layer. Finally, spoon the creamy mashed potatoes evenly over the top, smoothing them out gently to cover all the filling. Bake the assembled pie in the preheated oven for about 30 minutes, or until the top is beautifully browned and the dish is bubbling around the edges. Allow the pie to stand for 15 minutes before serving to let the layers set for easier slicing. TIP: You can prepare the lentils, sauce, and roasted vegetables a day ahead—just cool, cover separately, and chill. When ready to serve, assemble and bake as directed. This rest time lets the flavors meld beautifully, enhancing the final taste.