Leek, Mushroom, and Pea Potpie
Preheat oven to 400°F.
Melt the butter in a large pot over medium. Add the leeks and cook, stirring, 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, 3 minutes. Gradually whisk in 3 cups of the milk and simmer, whisking constantly, until thickened, 2 to 3 minutes. Stir in the peas, tarragon, salt, and pepper. Spoon the mixture into an 8-inch square baking dish.
Place the puff-pastry sheet over the filling, pressing the edges to seal. Brush with the remaining milk; cut slits into the top to release steam.