This recipe is an adaptation of an asparagus tart I enjoyed my first spring living in New England.
1. Adjust the oven rack to middle position and set the oven to 400 degrees. Line the chilled crust with parchment paper and fill with pie weights or dried beans. Blind bake the tart shell for 15 minutes. Carefully remove the parchment and the weights, prick the dough with a fork and return to the oven for an additional 5 minutes or until the bottom crust looks dry and flaky, but still pale. Brush with the lightly beaten egg.
2. In a heavy bottom skillet, heat the butter and olive oil over medium heat. Add the leeks and saute until completely softened, about 25 minutes, adjusting the heat as they cook to minimize browning. Set aside to cool.
3. Snap the ends off of the asparagus and peel each woody spear. Place into a colander or steaming basket over 1-2 inches of boiling water, cover and steam until the thickest part of the spear is easily pierced with a pairing knife, approximately 4 minutes for medium stalks. Set aside to cool.
4. Whisk heavy cream, egg yolks, and grated Parmesan until combined. Add the cooled leeks. Season with salt and pepper.
5. To assemble the tart, layer the asparagus in the tart shell and pour the custard over.
6. Bake until golden brown and the custard is set, but still slightly quivering in the center, about 30 minutes. Let cool at room temperature for 10-15 minutes and serve warm, or chill for later service.