Heat butter or oil in 10 inch cast iron skillet over medium heat.
Sauté mushroom and leek slices until soft and lightly browned. Season with salt and pepper.
Beat eggs and milk together in a mixing bowl. Pour over mushrooms and leeks and sprinkle cheese on top. Let cook for 8 or so minutes until the edges are set and can separate easily from the side of the pan.
Place in oven for 10-15 minutes. It’s done when the eggs are no longer runny and the center is firm. Do not overcook– check 5 minutes before it’s supposed to be done to play it safe.