Classic Chicken Potpie with Thyme

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.