Steam potatoes, covered, for 20-25 minutes, or until tender.
Let cool until they can be handled.
Peel and cut crosswise into thin slices.
In a 12 inch non-stick skillet, cook the leek in the butter over moderately-low heat, stirring, for 12-15 minutes, or until very soft.
Season to taste.
In a large bowl, whisk together eggs, ½ cup parmesan, ½ cup gruyere, chives, parsley and season to taste.
Whisk until combined well.
Stir in leek and potatoes.
In the cleaned skillet, heat the oil over moderate heat until hot but not smoking.
Pour in egg mixture, distributing potatoes evenly, and cook, without stirring, for 10-12 minutes, or until edge is set but center is slightly soft.
Put a plate, larger than the skillet, over the skillet and 'flip' frittata onto it. Gently slilde frittata back into skillet, now browned-side-up and continue cooking until other side has set, about 4 minutes longer.