1. Preheat the oven to 375°.
2. In a medium saucepan, cover the potatoes with water and bring to a boil. Boil the potatoes until tender, about 15 minutes. Drain and let cool slightly, then cut into 1/2-inch dice.
3. In a 10-inch nonstick ovenproof skillet, melt the butter. Add the leeks and cook over moderate heat until tender, about 4 minutes.
4. Add the potatoes and cook for 3 minutes longer. Season with salt and pepper.
5. In a large bowl, whisk the eggs with the milk and season with salt and pepper. Pour the eggs over the leeks and potatoes and cook for 5 minutes without stirring.
6. Dollop the fromage blanc or sprinkle the feta cheese over the leeks and potatoes.
7. Transfer the skillet to the oven and bake until the eggs are just set in the center, about 18 minutes. Slide the frittata onto a serving plate. Cut into wedges and serve.

Serves 8.