Preheat the oven to 375°F (190°C). Line a 9 x 13 inch (23 x 33 cm) baking pan with parchment paper using the “sling” method: cut two rectangles of parchment paper the width of the baking pan—one wider and one narrower—and overlap them, letting extra paper overhangs on all sides of the pan. To unmold the bars, you’ll simply pull on this overhanging paper and be done! Alternatively, you can simply use one large rectangle of parchment paper and fit it into the mold, creasing it in the corners and, again, letting extra paper overhang to pull out the bars later.
For the crumb layers: In the bowl of a food processor, add the flours, oats, sugar, baking powder, salt, and orange zest. Pulse a few times to combine. Add the cubed butter and pulse 15 times. Scrape down the bowl and stir the mixture using a spatula, then pulse 15 times. Add the egg, pulse 15 times, then scrape down the bowl and stir the mixture again. Process for a few seconds until the dough is crumbly.
Spoon two-thirds of the crumb mixture into the prepared parchment-lined baking pan. Using your hands, press the mixture down firmly to create the bottom crust. Set the remaining crumb mixture aside while you make the rhubarb filling.
For the rhubarb filling: In a large bowl, stir together all the filling ingredients. Distribute the rhubarb mixture evenly over the crust. Spread the remaining crumb mixture over the rhubarb filling.
Bake the rhubarb crumb bars for 45 minutes, or until top is slightly brown and juices are bubbling around the edges of the pan. Transfer to a wire rack and let cool completely, at least 1 hour.
SERVING: Pull on the overhanging parchment paper to get the bars out of the pan. Slice into squares or bars and enjoy as is, or serve with vanilla gelato or vanilla Greek yogurt.
STORAGE: To keep these rhubarb crumb bars crunchy, refrigerate the bars in an open container. Storing the bars in the refrigerator keeps them moist (they would dry up quite a bit if they remained at room temperature) and keeping them in an open container prevents the crumbs from absorbing too much moisture from the filling. An easy, perfect remedy to soggy crumb bars! You can keep the rhubarb crumb bars refrigerated for up to 1 week.
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