-(optional) For Pastry: =========== In a mixing bowl or processor, combine the flour, the salt, and sugar. Cut in 1/2 cup of butter until crumbly. Add just enough cold water so that they dough holds together in a ball. Seal in plastic wrap and refrigerate for at least 1 hour. For Filling: ============ Working in small batches, blanch the Swiss chard in boiling salted water. Drain, rinse under cold water, and thoroughly squeeze dry. Chop the chard finely and set aside. Heat 2 tablespoons of butter and oil in a large skillet. Add chilies, garlic, ginger and saute over medium heat for 1 minute without browning. Add the onion, and cook with stirring until translucent. Add the nutmeg, cumin and thyme and cook 1 minute longer. Add Swiss chard and stir until completely dry. Remove from heat, season to taste with lemon juice and salt, and set aside. To Assemble: ============ Roll out the dough on a lightly floured work surface to a very large circle 1/4 inch thick. Fold into quarters and carefully transfer the dough to a buttered 9-inch springform pan. Unfold the dough and gently fit it into the bottom of the pan, leaving a generous overhang. Brush the bottom and sides with some of the egg yolk mixture and set aside to dry for about 10 minutes. Preheat oven to 375 F. Spread half of the Swiss chard filling over the dough and arrange fish fillets on top. Cover evenly with remaining Swiss chard. Gently gather up the edges of the dough in the center, and twist it into a topknot, pinching the knot with thumb and forefinger. Leaving a knot 2 to 3 inches high, cut off the excess dough. Brush with the remaining egg yolk mixture and sprinkle with mustard seeds, and cut several slashes around the top. Place on a baking sheet and bake until golden, 1 to 1 1/4 hours. Cool completely on a rack before removing sides of the pan. Cut into wedges and serve at room temperature.