How to Make It
Preheat oven to 425°F. Cut rhubarb halves to fit a 9-inch springform pan. Place rhubarb strips on a baking sheet lined with parchment paper. Bake in preheated oven until rhubarb strips just begin to soften, 6 to 8 minutes. Let rhubarb cool on baking sheet 5 minutes; remove rhubarb, and let cool until ready to use. Reduce oven temperature to 325°F.
While rhubarb cools, beat egg whites in a large bowl with an electric mixer on high speed until stiff peaks form, about 2 minutes.
In a separate large bowl, beat egg yolks and granulated sugar with an electric mixer on high speed until thick and pale, about 1 1/2 minutes. Add vanilla, lemon zest, and salt, beating until well combined, about 30 seconds.
Grease 9-inch springform pan with cooking spray, and coat with almond flour. Fold one-third of the beaten egg whites and one-third of the almond flour into egg yolk mixture until just combined (do not over fold); repeat 2 more times. Pour batter into prepared pan.
Bake at 325°F until top of cake just starts to set up, about 25 minutes. Arrange rhubarb strips in a decorative pattern on top of cake, cutting in half crosswise if necessary; sprinkle with sanding sugar. Bake at 325°F until cake is golden brown and a wooden pick inserted in center comes out clean, 35 to 40 minutes. Transfer to a wire rack, and let cake cool in pan 10 minutes. Remove sides from pan, and let cool completely, about 40 minutes.