Step 1

Preheat the oven to 400 degrees F. Put the potatoes in a large saucepan, cover with cold water by 1 inch and season with salt. Bring to a boil, then reduce the heat and simmer until tender, 20 to 25 minutes.

Step 2

Meanwhile, fill a separate large saucepan with 6 cups water; add the bay leaves, thyme sprigs and 2 teaspoons salt. Bring to a boil. Add the carrots, rutabaga, turnip and leeks; reduce the heat to medium low and simmer until the vegetables are tender, about 10 minutes. Reserve 1 1/2 cups cooking liquid, then drain the vegetables. Discard the bay leaves and thyme. Pat the vegetables dry; set aside.

Step 3

Melt 2 tablespoons butter in a large skillet over medium-low heat. Add the mushrooms; cook until they release their liquid, 3 minutes. Increase the heat to medium; cook until the liquid is evaporated, 2 to 3 minutes. Sprinkle with the flour, coriander and nutmeg; cook, stirring, 1 minute. Whisk in the reserved cooking liquid and 3/4 cup half-and-half. Bring to a simmer; cook until thickened, 3 minutes. Stir in the carrot mixture, peas and lemon zest and juice. Return to a simmer, then remove from the heat. Season with salt and stir in the parsley.

Step 4

Drain the potatoes and let cool slightly. Return to the pot and add the curry powder and the remaining 4 tablespoons butter and 3/4 cup half-and-half. Season with 3/4 teaspoon salt and mash well.

Step 5

Spread the mushroom mixture in a 3-quart baking dish. Dollop the mashed potatoes on top; spread with the back of a spoon. Bake until bubbling around the edges and the topping is browned in spots, about 20 minutes. Let rest 10 minutes before serving.