Preheat the oven to 175C & line a baking sheet with parchment paper; set aside. Grease a 9-inch tart tin with butter & place on a different baking sheet; set aside.
Place the rhubarb on the first baking sheet with the parchment paper & sprinkle with the sugar.
Roast the rhubarb until tender, but so as it still has some strength & structure to it, about 15 minutes. Remove from the oven & leave to cool completely.
After removing the rhubarb from the oven, lower the temperature to 160C.
To make the financier, place the butter in a pan over a medium to high heat. Let the butter cook & sizzle until the liquid becomes a light brown colour & the solids on the bottom of the pan are a dark brown in colour, but not too brown! Remove from the heat when the foaming has subsided slightly & pour the brown butter in a cool bowl. This should yield about 90g of brown butter, which is all we need.
While the butter cools, sift the almond flour, plain flour & icing sugar into a large bowl. Add in the egg whites & beat (you can use a electric whisk or the plain old handheld kind, I opted for the later) until everything is just incorporated. As you do so be sure to keep scraping down the sides of the bowl to make sure you don’t miss anything.
By this point the brown butter should just be kind of warm, which is perfect. Slowly pour the butter & orange juice into the egg white mixture, doing so in batches until everything is all mixed up together & perfectly smooth.
Pour the financier batter into the tart tin. Layer the cooled strips of rhubarb over the batter until you have a pretty design. In hindsight, it would have been better if I’d cut my rhubarb into smaller strips to make it easier to eat after baking. Sprinkle the caster sugar evenly over the top of the financier.
Bake the financier until the batter is golden brown & a skewer inserted into the middle comes out with a few crumbs sticking to it, about 1 hour & 15 minutes. Remove from the oven & leave to cool completely.
To serve, lightly dust the financier with icing sugar.. Or if you’re like me, get carried away taking cool pictures & completely overdose the financier with icing sugar.