In a skillet, melt butter over medium heat. Add onions, carrots, and potatoes, and sauté until softened, about 5-7 minutes.

Stir in the cabbage and cook for an additional 3-4 minutes until wilted.

Sprinkle flour over the mixture and stir to combine. Gradually add beef broth, stirring until thickened.

Add shredded corned beef, Dijon mustard, thyme, and black pepper. Mix well and remove from heat.

In a mixing bowl, combine flour and salt. Cut in cold butter until the mixture resembles coarse crumbs.

Gradually add ice water, mixing until the dough comes together. Divide dough in half, shape into discs, and refrigerate for 30 minutes.

Preheat the oven to 375°F (190°C). Roll out one disc of dough and fit it into a pie dish.

Pour the filling into the crust. Roll out the second disc and place it over the filling. Seal and cut slits for steam.

Bake for 30-35 minutes or until the crust is golden brown.