Place whole wheat flour, all purpose flour and salt in a food processor and pulse a couple times to mix. Add cut up butter and continue pulsing until mixture has the consistency of coarse sand. (Alternatively you can use a pastry blender.)
Slowly add ice water into the running food processor until a dough ball forms. Use only as much water as needed.
Turn dough out onto a large piece of plastic wrap and shape into a disk. Wrap and chill for about half an hour.
In the meantime combine apple sauce, apple pie spice and maple syrup in a small bowl. Peel, core and thinly slice the apples and drizzle with the lemon juice.
Preheat your oven to 400 degrees F. Sprinkle some flour onto a clean surface and on top of the chilled dough disk. Roll out to about ⅛ inch thickness. Place on a (lined) baking sheet.
Spread apple sauce onto crust, starting about 2 inches from the edge. Arrange apple slices in a circular pattern. Sprinkles with walnut pieces and sugar and fold over the edges.
Bake galette for 35-40 minutes until edges are golden-brown.
Combine powdered coconut sugar with only as much maple syrup as needed to reach desired consistency. Drizzle over galette once it has cooled a bit.