Preheat the oven to 425°F (220°C).

In a large skillet, melt 2 tablespoons of butter over medium heat. Add diced onions, carrots, and potatoes; sauté until the vegetables are tender.

Stir in the shredded corned beef, shredded cabbage, beef broth, Dijon mustard, thyme, black pepper, and salt. Cook until heated through.

In a separate bowl, combine 2 tablespoons of flour with the sautéed mixture to thicken the gravy.

For the crust, combine 2 cups all-purpose flour, 1 teaspoon salt, and 1 cup chilled cubed butter in a bowl. Mix until crumbly, then add ice water gradually to form a dough.

Roll out the dough and fit it into a pie dish. Fill with the corned beef and cabbage mixture.

Roll out a second dough layer for the top and place it over the filling. Seal the edges and make slits for steam to escape.

Bake in the preheated oven for 30-40 minutes or until the crust is golden brown.