1. Turn out pate brisee onto a lightly floured surface. Roll dough into a 13-inch round, about 1/8 inch thick. Press dough into bottom and up sides of a 10-inch springform pan with a removable bottom. Using a paring knife, trim dough to form 1 1/2- to 2-inch-high sides. Transfer to refrigerator, and chill for 30 minutes.

2. Preheat oven to 375 degrees. Line dough with parchment paper, leaving a 1-inch overhang. Fill with pie weights or dried beans. Bake until edge just begins to turn golden brown, about 25 minutes. Remove parchment and pie weights. Bake until pale gold, about 15 minutes. Transfer pan to a wire rack, and let cool completely.

3. Meanwhile, heat oil in a large saute pan over medium heat. Add leeks, and cook, stirring often, until softened, about 10 minutes. Add wine and Swiss chard leaves, and cook until liquid has evaporated and chard has wilted, about 5 minutes.

4. Melt butter in a medium saucepan. Whisk in flour until incorporated. Slowly whisk in milk, and bring to a boil. Cook until thickened, about 3 minutes.

5. Combine leek mixture, milk mixture, mustard, and egg. Season with salt and pepper, and stir.

6. Spread leek mixture into tart shell, and sprinkle with Gruyere. Bake until top is golden brown, about 35 minutes. Let cool slightly, and serve warm.