Step 1

Cut 6 tablespoons of the unsalted butter into 6 pieces and place on the counter and let sit at room temperature until softened. Meanwhile, peel and cut 2 1/2 pounds Yukon gold potatoes into 1 1/2-inch chunks. Place in a large saucepan or pot and add enough cold water to cover by about 1 inch. Add 1 tablespoon of the kosher salt and bring to a boil over medium-high heat. Reduce the heat to maintain a simmer and cook until the potatoes are knife tender, 15 to 25 minutes. (This is a good time to cut the vegetables for the filling.)

Step 2

Drain the potatoes. Pass the potatoes through a ricer into the now-empty pot, or return the potatoes to the pot and mash with a potato masher until smooth.

Step 3

Place 1/3 cup heavy cream, 2 large egg yolks, 1/4 teaspoon freshly grated nutmeg, and the remaining 1 teaspoon kosher salt in a liquid measuring cup or small bowl and whisk to combine. Add the cream mixture and softened butter to the potatoes. Stir until the butter is melted and the mixture is combined. Taste and season with more kosher salt as needed. Cover to keep warm.

Step 4

While the potatoes cook, arrange a rack in the middle of the oven and heat the oven to 425ºF. Peel and thinly slice 2 medium shallots (about 3/4 cup). Peel and dice 3 medium carrots (1 cup) and 2 medium celery stalks (2/3 cup). Remove the core from 1/2 large fennel bulb, then thinly slice (about 1 1/4 cups). Mince 2 garlic cloves. Pick the leaves from 4 fresh tarragon sprigs, then coarsely chop (about 2 tablespoons). Dice 1 1/2 pounds cooked chicken (about 4 cups).

Step 5

Melt 4 tablespoons unsalted butter in a large skillet over medium heat. Add the shallots and cook, stirring occasionally, until softened and starting to brown, about 5 minutes. Sprinkle with 1/3 cup all-purpose flour. Cook, stirring constantly, until the flour smells nutty and darkens slightly in color, about 2 minutes.

Step 6

Add the carrots, celery, fennel, garlic, and 1 teaspoon kosher salt. Pour in 1 (32-ounce) carton low-sodium chicken broth and stir to combine. Bring to a boil.

Step 7

Reduce the heat to maintain a simmer. Cook, stirring occasionally, until the vegetables are almost tender and the sauce is thickened enough to coat the back of a spoon, about 15 minutes. Remove the pan from the heat.

Step 8

Add the tarragon, chicken, 1 cup frozen peas, 1 tablespoon white wine vinegar, and 1/2 teaspoon black pepper, and stir to combine. Taste and season with more kosher salt and black pepper as needed.

Step 9

Transfer the filling to a 2 1/2 to 3-quart baking dish (such as a 9x13-inch) and spread into an even layer. Dollop the potatoes over the filling with a flexible spatula, then spread the potatoes evenly over the filling, going all the way to the edges of the baking dish. Dice the remaining 2 tablespoons unsalted butter and scatter over the potatoes.

Step 10

Bake until the potatoes are browned in spots and the filling is warmed through and bubbling, about 25 minutes. Garnish with 1 tablespoon finely chopped fresh chives or tarragon leaves if desired. Let cool for 10 to 15 minutes before serving.