To a food processor fitted with a metal blade add the flour, butter, 1 tablespoon sugar, salt and nutmeg. Pulse until the mixture resembles coarse meal, about 5 to 10 seconds. Add the orange zest. With the processor running, pour enough of the water through the tube in a steady stream until the dough forms a ball. You may not use all the water. Do not over mix.
Dust a clean work surface with flour.
Remove the dough from the processor, turn out onto the floured surface and knead for 1 minute. Form the dough into a ball, wrap in plastic wrap and refrigerate for 30 minutes.
Preheat oven to 375°F/190°C and line a large baking sheet with parchment paper (or an 11-inch/28 cm flat cast iron skillet)
To a small bowl add the marmalade and nutmeg, mix well. If the marmalade is to hard to mix, warm it a little in the microwave. Set aside.
Roll dough out to a 13 ½ inch (35 cm) round. Spread the 3 tablespoons marmalade all over the pastry, leaving a 1 ½ inch/4 cm border. Leaving a little to add after baking.
Arrange the orange slices in an even layer. Fold the pastry border over to the edge of the fruit.
Brush the pastry edge with the egg wash and sprinkle lightly with the ½ teaspoon sugar (optional).
Bake for 25-30 minutes or until golden brown (all ovens vary).
When the galette is done, remove from the oven and brush the oranges with the marmalade. Transfer to a cooling rack and allow to cool completely.