The Leeks in this recipe melt into a caramelized crust under the potatoes and cheese. A slice of this with a heart salad is a great lunch, or serve it with poached eggs for brunch. You can make this using 1 C. sliced leeks and 1 lb. of potatoes in a small skillet, 2 C. and 2 lbs. in a medium skillet, or 3 C. and 3 lbs. in a large skillet.
Preheat the oven to 375.
Cut off the leaves off the leek or leeks and discard. Cut the shank of each leek in half lengthwise at least half way down from the top and rinse it well under running water.
Finish cutting the leeks in half, then cut the halves into 2 inch pieces and place cut side down. Thinly slice each piece into tiny strips.
Saute the leeks in olive oil with salt on low heat in a cast iron skillet until tender.
In the meantime, wash potatoes and slice into the largest possible rounds (you can use a mandoline to do this) as thinly as possible.
When the leeks are soft, sprinkle black pepper and fresh chopped rosemary, sage and/or thyme over them.
Arrange the potato slices in overlapping layers over the leeks, drizzling or brushing each layer with a little more olive oil and salting it.
Top the gratin with 1/2 C. grated Gruyere cheese per pound of potatoes and sprinkle with more fresh herbs.