1 tablespoon unsalted butter
2 large leeks, white and pale green parts only, cleaned and sliced
3/4 cup grated Gruyere cheese
1/4 cup grated Parmesan cheese
1 cup whipping cream or half-and-half
2 tablespoons minced fresh tarragon or 1 tablespoon dried, crumbled tarragon
salt and freshly ground black pepper
Preheat oven to 375 degrees F. Lightly grease an 8-inch square baking dish. Heat butter in medium skillet over medium heat. Add leeks and sauté until tender, about 5 minutes. Spread leeks evenly on bottom of baking dish. Combine cheeses in a bowl and toss to mix. Spread all but 1/4 cup of the cheeses evenly over leeks. In a large bowl, whisk together eggs, cream and tarragon, with salt and pepper to taste. Pour egg mixture into dish and bake until top is golden and center is set, about 30 minutes. Take dish from oven, sprinkle reserved cheese over, and continue baking just until cheese melts, 3 to 5 minutes longer. Let sit for a few minutes, then cut into squares to serve.
Serves 2 to 4.