Preheat oven to 375°F. Add diced potatoes to a large pot; cover with water and bring to a boil. Boil until tender, about 10-15 minutes, drain and set aside.
Meanwhile, in a large saucepan over medium heat add olive oil. Once oil is shimmering, add chopped onions. Stir until soft, about 3-4 minutes and then add garlic and butter. Once melted, add flour and whisk until mixture is thick; cook 1-2 minutes. Add spices; stir and then slowly add in chicken stock (or broth) and milk, whisking as you add the liquids. Simmer over medium-low heat until mixture thickens slightly.
Add chicken, vegetables and potatoes to cream sauce; stir to coat and set aside.
Place one prepared pie crust in a 9-inch pie pan (use a deep-dish pie pan if you have one) and lightly brush with egg white. Use a fork to pierce the bottom crust a few times. Bake 5 minutes, remove, and pour in chicken mixture. Top with second crust and crimp edges to seal. Use a knife to cut slits in the top crust; brush with egg whites. Bake 35-40 minutes or until the crust is golden brown.
Remove and let rest 5-10 minutes before serving.