To make the dough mix flour, sugar, and salt in a mixing bowl. Add cold diced butter and toss in the flour. Pinch the butter cubes in-between your fingers until you have little flakes of butter, then begin to add your water 1 tbsp at a time until you have a shaggy dough. Turn it out onto a surface and using either your hands or a bench scraper fold the dough over and onto itself until its a cohesive dough. Cover and place in the fridge for 1 hour prior to rolling
To make the frangipane add sugar and soft butter to a bowl and cream together until light and fluffy. Add the almond flour and mix to combine. Next add egg, vanilla, and brandy. Mix until combined and reserve in the fridge.
A large stalk of rhubarb will make this part easier since the larger, thicker size will yield you larger ribbons and coverage. You want to slice the rhubarb in long, wide ribbons. Using a mandolin or a very sharp knife carefully slice rhubarb 1/8th of an inch thick. You should have around 20 of these rhubarb ribbons to cover the top of the galette. Set aside
Roll out your rested dough on a floured work surface until its 1/4th of an inch thick and use an 8 inch plate to place on top cut out a perfect circle. Spread your frangipane evenly in the center leaving 1 inch empty around the border. Grab your sliced rhubarb and begin to lay them overlapping on top, kind of like shingles on a roof, you can now grab a pair of scissors and cut away excess rhubarb around the edges. Grab the outside empty borders of dough and fold it inwards over the filling until it’s sealed around the exterior completely. Place on a small sheet tray and place in the fridge for 1 hour.
preheat oven to 375f
pull out your galette and brush the outside with egg wash, then sprinkle over Demerara sugar, bake for 30-40 minutes until golden