
This comforting chicken pot pie is packed with tender chicken, sweet vegetables, and a rich mustard-thyme sauce under a flaky puff pastry lid. It is a classic family dinner that feels cozy, hearty, and well worth the bake.
Heat 1 tbsp of the vegetable oil in a large oven-safe casserole or deep skillet over medium-high heat. Season the chicken lightly with salt and pepper, then brown half of it for 3 to 4 minutes per side. Transfer to a plate and repeat with the remaining chicken using the rest of the oil.
Reduce the heat to medium. Add the chopped onions, celery, and carrots to the pan and cook for 8 to 10 minutes, stirring often, until softened and lightly golden. Scrape up any browned bits from the bottom of the pan for extra flavor, then transfer the vegetables to the plate with the chicken.
Melt the butter in the same pan. Stir in the flour and cook for 1 to 2 minutes, stirring constantly, until the mixture turns pale golden and smells slightly nutty.
Gradually pour in the milk, starting with a small splash and whisking well after each addition to prevent lumps. Once all the milk is added, whisk until the sauce is smooth and thickened.
Crumble in the stock cube and stir in 250 ml water. Return the chicken and vegetables to the pan, then add the Dijon mustard and thyme. Bring to a gentle simmer, cover, and cook over low heat for 15 to 20 minutes, stirring occasionally, until the chicken is fully cooked and tender.
Preheat the oven to 200C / 180C fan / gas 6. Use two forks to shred the chicken into bite-sized pieces directly in the sauce, then taste and adjust the seasoning with salt and pepper. Let the filling cool slightly if needed before topping with pastry.
Spoon the filling into a pie dish. Brush beaten egg around the rim, then lay the puff pastry over the top. Trim away any excess, press the edges to seal, and crimp if desired.
Brush the pastry all over with the remaining beaten egg. Cut a few small slits in the top to let steam escape, then bake for 35 to 40 minutes until the pastry is deep golden and the filling is bubbling around the edges. Let the pie rest for 10 minutes before serving.
For a make-ahead option, prepare the filling up to 2 days in advance and chill before topping with pastry. If the sauce seems too thick, loosen it with a splash of milk or stock; if it seems thin, simmer uncovered for a few extra minutes before assembling the pie. You can also swap chicken thighs for boneless skinless chicken breasts, but thighs stay juicier during baking.