1 tablespoon whole wheat pastry flour, plus extra for rolling
Glaze & Garnish
1 tablespoons heavy cream
3 tablespoons raw/demerara sugar
In a medium bowl, whisk together the all-purpose flour, pastry flour, sugar, and salt.
Using the large-hole side of a box grater, grate butter into bowl. Toss with a wooden spoon until all of the butter is coated in flour and no large clumps remain.
Add 5 tablespoons ice water and work in gently with hands, beginning to bring dough together in a ball. Add another 1-2 tablespoons ice water as needed until the dough forms a ball that is neither dry nor sticky. Be careful not to overwork the dough. Wrap in plastic wrap and refrigerate for about 45 minutes.
Meanwhile, in a separate bowl, toss together the rhubarb, apple, sugars, cornstarch, lemon juice, and cinnamon. Lay out a sheet of parchment paper to use for rolling and flour parchment generously.
Flouring dough as needed, roll out to a 12" circle. Dust with 1 tablespoon flour and layer fruit mixture on top, leaving an approx. 2" border clear around the outside. Lift parchment and transfer to a baking sheet. Freeze for about 10 minutes. Meanwhile, preheat oven to 400ºF.
Remove galette from freezer and carefully fold and pleat edges over outer border of filling. Brush crust with cream and sprinkle galette all over with Demerara sugar. Bake in preheated oven for 30-35 minutes, until crust is golden and fruit is tender. Allow to cool to room temperature before serving. Leftovers can be covered in plastic wrap and stored in the refrigerator.
*I suggest using a crisp, somewhat sweet apple here; I like Pink Lady.