Heat the cast iron skillet over medium heat and add in the butter. Then add in the leeks and the broccoli and cook for 5 minutes. Stir often to avoid burning.
Whisk together the eggs and the milk until well combined.
Once the veggies are cooked, add in the rosemary, and then spread the vegetables evenly through the pan. Pour the egg mixture into the skillet and carefully level it out. Sprinkle the cheese all over and then carefully transfer the skillet to the oven.
Cook for 15 minutes. The egg should be set and the cheese should have started to brown. Remove from the oven and enjoy!
Notes
You can use whatever vegetables you like in the frittata. Any cheese will work on top. A strong flavoured cheese pairs well with the broccoli.