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Preheat the oven to 350°F. Cut a circle of parchment paper 8 inches in diameter.
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Lay out the rhubarb stalks parallel to each other over the parchment circle, then cut them so they fit just inside the circle, reserving any excess rhubarb.
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Grease an 8-inch cake pan with butter, line the bottom of the cake pan with the parchment circle, and grease it also. Sprinkle the 2 tablespoons sugar on the bottom and sides of the cake pan.
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Fit the trimmed rhubarb pieces, reddest side down, along the bottom of the pan to create one solid rhubarb layer. Chop any remaining rhubarb and cook in a saucepan over medium-low heat with 1/2 tablespoon water, stirring occasionally, until soft, 10 to 15 minutes. Using a spoon, mash the softened rhubarb until it’s broken down, then reserve 1/2 cup rhubarb puree. (Save the rest for another use.)
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In a large bowl, mix together the brown rice flour, almond flour, baking powder, baking soda, salt, and lemon zest.
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In a medium bowl, whisk together the remaining 3/4 cup sugar, the eggs, vanilla, olive oil, and the reserved rhubarb puree. Pour the wet ingredients over the dry ingredients and whisk to combine.
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Pour the batter over the rhubarb in the cake pan and spread evenly. Bake until a toothpick inserted into the center comes out clean, 50 to 55 minutes. Cool in the pan for 15 minutes, then invert onto a cooling rack. Transfer to a cake plate, bottom side up, dust with powdered sugar, and serve.