Topping
3 pounds russet potatoes, unpeeled
3 pounds Yukon Gold potatoes, unpeeled
1/2 cup (1 stick) unsalted butter, cut into 1/2-inch cubes
1 1/2–2 cups whole milk, warmed
Kosher salt
Filling
1 ounce dried porcini mushrooms
3/4 cup brown or French green lentils
6 garlic cloves, divided, plus 2 Tbsp. 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-inch pieces peeled fall vegetables (such as squash, turnips, carrots, and parsnips)
1 cup frozen pearl onions, thawed, halved
2 4-inch rosemary sprigs
2 cups bite-size pieces mixed fresh mushrooms
1/4 cup chopped mixed fresh herbs (such as parsley, chives, and sage)