• Pre-heat the oven to 350°F.

Prep the leek

• Slice the dark green tops off the leek and discard, use only the light green and white part. Slice in half lengthwise and run under cold water to remove and grit or sand. Lay flat on a cutting board and chop into thin pieces.

Grate the cheese

• With a hand grater or food processor, grate the cheeses.

Cook the leeks

• In a 10–12 inch well-seasoned cast iron skillet or oven-safe sauté pan, melt the butter over medium to medium-low heat. Add the leek and cook 3–4 minutes until softened. Don't brown it. Turn heat down if needed.

Make the frittata

• Add the chopped ham to the skillet, stirring to combine and warm through. Sprinkle the Cheddar and Gruyère evenly over the top, then the parsley.
• In a medium bowl, whisk the eggs, half-and-half, salt, and pepper until smooth. Pour the custard evenly over the filling. Top with Parmesan for a golden crust after baking.
• Transfer the skillet to the oven and bake 30–40 minutes, until the center is just set and the top is lightly browned. Let the frittata rest 5–10 minutes before slicing. Serve warm or at room temperature.