Preheat oven to 400°F. Heat oil in a 10-inch cast iron skillet over medium until shimmering. Add onion, carrots, and potatoes; cook, stirring frequently, over medium until carrots and potatoes ...

Per Drummond's instructions, the pie is done when the crust turns brown and the filling gets bubbly, which should take about 25 to 30 minutes in the oven.