Ingredients
* 3 tablespoons unsalted butter, at room temperature
* 2 medium white onions, thinly sliced
* 2 medium leeks, sliced lengthwise and thinly sliced (white part only)
* 1 teaspoon finely chopped fresh thyme leaves
* 2 teaspoons finely chopped fresh marjoram leaves
* 1 tablespoon finely chopped fresh Italian parsley
* 1/2 cup crème fraîche
* Basic Pie Dough, refrigerated
* 1 large egg yolk
* 1 teaspoon cold water
Method
1. Melt 1 tablespoon of the butter in a large frying pan over medium heat. When butter is foaming, add 1/3 of the onions and leeks. Season well with salt and freshly ground black pepper. Let onions and leeks cook until they are caramelized and golden brown, about 10 minutes. Season again with salt and pepper. Remove from the pan and reserve in a mixing bowl. Cook remaining onions and leeks in two more batches, using 1 tablespoon butter per batch, until they are all caramelized.
2. Combine herbs and crème fraîche with onion-and-leek mixture, and check the seasoning. Add more salt and pepper as necessary, and let mixture rest until it is cooled, about 5 minutes.
3. Heat the oven to 220C and arrange a rack in the middle. Lightly flour a work surface and roll dough out into a round approximately 11 inches in diameter and 1/8 inch thick. Cut dough into a 10-inch round (an overturned dinner plate and a paring knife work great). Lay dough on a baking sheet and evenly spread filling throughout, leaving a 1-inch border. Fold the perimeter of the dough (the outer 1 inch) over the filling, and flute the edge by pinching it between your forefingers.
4. Make an egg wash by whisking egg yolk with cold water. Brush outer edge of tart with the wash. Place on the middle rack of the preheated oven and bake until the bottom of the crust is golden, about 20 to 25 minutes.