Preheat the oven to 350°F. Cut a circle of parchment paper 8 inches in diameter.
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.
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.
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.)
In a large bowl, mix together the brown rice flour, almond flour, baking powder, baking soda, salt, and lemon zest.
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.
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.