Blanch the swiss chard in boiling water for a minute; drain and squeeze dry. Transfer to a mixing bowl and add the basil, cheeses, and 4 of the eggs. Season with salt and freshly ground black pepper and add half of the saffron and soft butter.
Fill the prebaked tart shell and bake for 15 minutes. Beat remaining egg with remaining saffron and spoon this over the surface of the tart. Bake for 20 minutes longer or until puffy.