1 sheet frozen puff pastry (half of 17.3 ounce package), thawed
2 tablespoons (1/4 stick) butter
3 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 or whole milk
3 large eggs
2 large egg yolks
1 teaspoon salt
1/4 teaspoon ground black pepper
Pinch of ground nutmeg