Preheat your oven to 350°F (175°C).

Melt the butter in an oven-safe skillet over medium heat.

Add the sliced leeks and cook until they are soft and translucent, about 5 minutes.

In a mixing bowl, whisk together the eggs, salt, and pepper until well combined.

Pour the egg mixture over the sautéed leeks in the skillet and stir gently to combine.

Sprinkle the shredded cheese over the top of the egg and leek mixture.

Cook the frittata on the stove for about 5 minutes, until the edges start to set.

Transfer the skillet to the preheated oven and bake for 15-20 minutes, or until set and slightly puffed.

Remove from the oven and let cool for a few minutes. Slice and serve warm.