Directions

1. Place a rack in upper third of oven; preheat to 325°. Heat oil and butter in cast iron skillet over medium heat. Add leeks and ham; cook, stirring often, until softened, about 5 minutes. Add mushrooms and ham; cook, stirring often, until mushrooms are softened and all liquid has evaporated, 8-10 minutes. Ham should also be nice and browned by this time as well.
2. Meanwhile, whisk eggs, whipping cream, in a large bowl; mix in ½ cup cheese. Season with salt and pepper.
3. Increase the heat to medium-high and add a drizzle of oil to the skillet. Pour in the egg mixture over the contents, shaking the pan to evenly distribute mixture. Cook the frittata, without stirring, until its edges begin to set, about 5 minutes.
4. Transfer skillet to oven. Bake frittata until golden brown and center is set, about 20 minutes I’d say but keep an eye on it. At this point, crank the oven up to high broil and sprinkle the remaining cheese over the top. Leave in for another 2-5 minutes to melt and brown the cheese.