Preheat the oven to 400 °. Grease a 9-inch pie plate with nonstick cooking spray.
Boil the potatoes in a large pot of salted water. Cook until tender. Drain the potatoes and place in a large mixing bowl.
Mash the potatoes thoroughly with a potato masher. Add the butter and mash it in well.
In a medium skillet, heat the oil until shimmering. Add the leeks and saute over low heat until the leeks are tender, about 10 minutes.
Add the leeks and 1 cup shredded cheese to the potatoes. Add enough milk to create a thick puree.
Spoon this mixture into the prepared baking dish.
Top the potato mixture with the remaining cup of cheddar cheese. (Can be covered and refrigerated at this point, if making ahead.)
Bake for 25 minutes or until the potatoes are hot and the cheddar top has melted and started to turn golden brown. (If cooking a dish that has been refrigerated, you may need to increase the time 10 to 15 minutes.)
Note: Promptly store leftovers in an airtight container in the refrigerator, for up to 4 days. Nutritional values are approximate and based on ⅛ the recipe.