A "deep pie dish" refers to a baking dish that is specifically designed to make pies with a lot of filling. It has higher sides and can hold more ingredients compared to regular pie dishes. Full definition
A kind of crustless quiche in which flavorful filling ingredients are piled into a deep pie dish and a milky - eggy batter is poured over. (shockinglydelicious.com)
If you are not using cast iron, or if you don't want to bake your fennel frittata right in the skillet, you can butter or oil a deep pie dish or other casserole pan and pour in the frittata ingredients. (healthygreenkitchen.com)