In a large cast iron skillet, heat 1 tablespoon oil over medium heat. Add ground lamb; cook until browned, breaking it up with a wood spoon as it cooks. Remove lamb from skillet; set aside. 

Using the same skillet, heat remaining oil. Add in celery, carrots and leeks. Cook until vegetables start to soften, stirring often. Add garlic; and continue cooking for 1 more minute. Add cooked lamb, potatoes, beef stock, curry and salt; stir until all ingredients are combined. Bring mixture to a simmer; cook until potatoes are tender. Add in peas; cook for 1 minute. 

In a small bowl, whisk together water and cornstarch until a murky liquid is achieved. Add slurry mixture to prepared lamb mixture; stir over medium-low heat until everything thickens, about 5 minutes. Remove from heat. 

On a well-floured cutting board,  roll out puff pastry until it’s large enough to cover the skillet. Set pastry over the skillet, pressing down on edges. Give pastry 3 or 4 slits in the top with a sharp knife. Using a pastry brush, brush top of pastry with egg. 

Set prepared pie on a large rimmed baking sheet; place in center of the oven. Bake at 400°F for 20 minutes, or until pastry is golden brown. Serve warm.