Leek, Mushroom & Pea Potpie

2 T. salted butter
4 leeks, light green parts only, washed thoroughly and thinly sliced
3 carrots, peeled and cut into 1/2-inch thick slices
14 oz. shitake mushrooms, stems removed, quartered
1/2 c. dry white wine
1/3 c. all-purpose flour
3 c. plus 1 T. whole milk, divided
1 c. frozen peas
1 T. chopped fresh tarragon
2 T. kosher salt
1/4 t. black pepper
1/2 of a 17.3-oz. package frozen puff-pastry sheets, thawed

Melt butter in a large pot over medium heat. Add the leeks and cook, stirring, for 5 minutes. Add the carrots and mushrooms and cook 5 minutes. Add the wine and cook 2 minutes. Sprinkle with the flour; cook, stirring constantly, for 3 minutes. Gradually whisk in 3 cups of the milk and simmer, whisking constantly, until thickened, about 2-3 minutes. Stir in the peas, tarragon, salt, and pepper. Spoon mixture into an 8" square baking dish.

Place the puff-pastry sheet oven the filling, pressing the edges to seal. Edges should be inside the dish, not fluted up like a pie crust. Brush with the remaining milk; cut slits in the top to release steam.

Bake at 400 degrees until the pastry is golden brown and the filling is bubbly, 30-35 minutes. Let stand for 15 minutes before serving.

Serves 6.

Adapted from Real Simple magazine, Apr. 2016, p. 190.