Delicious corned beef and cabbage, in pot pie form! Starts with cooked corned beef and store-bought puff pastry, so easy and a great use for your leftover corned beef. You can make the filling ahead and refrigerate.
In a large Dutch oven or pot, melt butter over medium heat. Add potatoes, carrot and cabbage and cook, stirring, for about 5 minutes. Add flour and cook, stirring, for 1 more minute. Add chicken broth, milk, mustard, thyme leaves and salt and pepper. Stir to combine. Bring mixture to a boil, then reduce heat to simmer. Simmer 15-20 minutes, or until vegetables are tender. Stir in corned beef and cheese an cook, stirring until cheese has melted. Taste and adjust seasoning as necessary. Remove from heat and allow to cool 15-20 minutes. You don't want to place the puff pastry on hot filling.
*You can make ahead to this point. Refrigerate and bake off with the puff pastry topping later. Pies may take an extra few minutes in the oven as they are starting from cold. If necessary, loosely cover the puff pastry with foil during the last few minutes of baking, to avoid over-browning.
Preheat oven to 425F and prepare a baking dish/skillet (or individual dishes/skillets). Place on a baking sheet and spoon filling in. Allow to stand to cool further while you prepare the puff pastry.
Cut puff pastry sheets to fit your baking dish or dishes. Cut several slits in the middle to allow steam to vent. Combine egg with water and brush top(s) of puff pastry pieces.
When filling has cooled sufficiently, place puff pastry pieces on top and bake in preheated oven for about 20 minutes, or until puff pastry is quite golden and filling is bubbling. Allow to stand 5 minutes before eating.