1 sheet frozen puff pastry (half of 17.3-ounce package), thawed (I used a basic tartdough instead) 2 tablespoons (1/4 stick) butter 3 large leeks (white and pale green parts only), coarsely chopped 1 teaspoon dried thyme
1/2 bunch Swiss chard, ribs removed, leaves chopped (about 2 1/2 cups) 1 1/4 cups whipping cream (I used whole milk) 3 large eggs 2 large egg yolks 1 teaspoon salt 1/4 teaspoon ground black pepper Pinch of ground nutmeg
Roll out pastry on floured work surface to 12-inch square. Transfer to 9-inch-diameter glass pie dish. Trim overhang to 1 inch. Fold under; crimp edges. Cover; chill.
Melt butter in large nonstick skillet over medium-low heat. Add leeks and thyme. Sprinkle with salt and pepper. Cover; cook until leeks are very tender but not brown, stirring often, about 10 minutes. Add chard; saute until wilted, about 2 minutes. Remove from heat; cool.
Position rack in bottom third of oven; preheat to 425°F. Whisk cream and next 5 ingredients in large bowl. Mix in cooled leek mixture. Pour filling into crust.
Bake tart 15 minutes. Reduce heat to 350°F and bake until filling is puffed and just set in center, about 15 minutes longer (this took my oven about 10 minutes longer). Transfer to rack; cool 10 minutes.