Heat buuter and oil in a 10 inch cast iron skillet over medium heat.
Saute leek slices until sft and barely browned. Season with salt and pepper.
Meanwhile, beat eggs and milk together in a mixing bowl. Pour over leeks and sprinkle with cheese. Let cook for about 5 minutes, until edges are just set. Stick in oven for 8 minutes. Frittata should be just set throughout.