If you like, you can use a 2-quart baking dish to make one big potpie: Roll out the pastry so it's 1 inch wider than the dish, cut vents, and bake for 45 minutes to 1 hour.
- 4 tablespoons unsalted butter
- 1 medium onion, cut into medium dice
- 1 large carrot, cut into medium dice
- 1/2 cup all-purpose flour, plus more for work surface
- Coarse salt and ground pepper
- 4 cups low-sodium chicken broth
- 3 cups cooked chicken, cut into 1-inch pieces (1 pound total)
- 1 cup frozen peas
- 1 1/2 teaspoons chopped fresh thyme leaves
- 1 sheet frozen puff pastry, thawed
- 1 large egg yolk
In a medium saucepan, melt butter over medium. Add onion and carrot and cook until onion softens, about 6 minutes. Add flour and 1/2 teaspoon salt; cook, stirring frequently, until mixture is pale golden, has a slightly nutty aroma, and is the texture of cooked oatmeal, about 5 minutes.
Whisking constantly, add broth. Bring to a boil, stirring frequently, until mixture thickens, about 8 minutes. Reduce to a simmer and cook 10 minutes. Stir in chicken, peas, and thyme; season with salt and pepper. Divide mixture among four 12-ounce baking dishes; refrigerate until room temperature, about 20 minutes.
Preheat oven to 375 degrees. On a lightly floured work surface, roll pastry to a 1/8-inch thickness. Cut into 4 equal squares, 1 inch larger than dishes; with the tip of a sharp knife, cut vents into pastry. In a small bowl, lightly beat egg yolk with 1 teaspoon water; top potpies with pastry and brush with egg wash. Refrigerate 15 minutes. Bake until pastry is deep golden and juices are bubbling, about 35 minutes.