2Assemble the mixer with the glass bowl and scraper paddle. Add the flour, powdered sugar, ground almond and butter to the bowl. Slowly turn the mixer to FOLDING/KNEADING setting and mix until it resembles a sand like texture.
3Add the egg yolk and gradually add the ice water (if needed) until pastry forms.
4Remove the pastry from the bowl and place it on a clean surface. Shape into a flat disk. Cover and refrigerate for 30 minutes.
5To make the crumble, put the sugar, almonds, cinnamon, oats and butter in a food processor and pulse until mixture starts to combine. Set aside.
6To make the filling, toss the rhubarb in the sugar and set aside.
7Roll the pastry between sheets of parchment paper to form a 14-inch round. Remove the top sheet of paper and transfer the pastry (on the paper) to a baking sheet. Scatter half of the crumble mixture over the center of pastry round. Top with the apple and rhubarb mixture. Sprinkle with remaining crumble mixture.
8Bring the edges of pastry up over filling, leaving top open. Brush the pastry with the egg white and sprinkle with the demerara sugar.