Preheat the oven to 400°F (200°C).
Prepare the filling by sautéing the onions until they're translucent.
Mix the shredded corned beef, potatoes, peas, mustard, Worcestershire sauce, thyme, and cream in a large bowl.
Season the mixture with salt and pepper to taste.
Roll out the pastry dough on a floured surface and line a pie dish with half of it.
Fill the pie with the corned beef mixture and top with the remaining pastry.
Cut slits in the top crust to allow steam to escape.
Bake in the preheated oven for 35-40 minutes or until golden brown.
Remove from the oven and let cool for 10 minutes before serving.