Position rack in bottom third of oven; preheat to 425°F.
Roll out pastry on a floured work surface to 12-inch square. Transfer to a 9-inch-diameter glass pie dish. Trim overhang to 1 inch. Fold under; crimp edges. Cover; chill.
Melt butter in a 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.
Whisk cream, eggs, egg yolks, salt, pepper, and a pinch of nutmeg in a 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.