Transfer the dough to a floured work surface and roll the dough out into a disc and chill.
In a bowl, toss the apples, walnuts, cinnamon, sugar, cornstarch, salt, and juice from the orange (reserving 1 tablespoon for crust).
Spread ½ cup of orange marmalade on top of the crust, leaving about 2 inches around the edges bare.
Mound the apples in the center of the crust and spread them our evenly, leaving 2-3 inches of crust edges bare.
Zest as much orange zest as you would like on top of the apples.
Fold bare edges up and over some of the apples, leaving the center mound without crust. (Video examples this easy procedure).
Mix 1 tablespoon of reserved marmalade with 1 tablespoon of orange juice and brush across all of the upper crust. Finish with a sprinkle of sugar, across the top.
Place in a 400 degrees, preheated oven for 10-minutes. Turn down the oven to 350 and continue baking until the crust is golden, about 20-minutes
Can be served with ice cream on top of the hot Galette, or a scoop of mascarpone!