Sift together the almond flour, sweet rice flour, oat flour, and tapioca starch in a medium bowl and set aside.
Place the white chocolate in a microwave-safe bowl and microwave for 30 seconds. Stir, then microwave for 30 seconds more. Stir until melted and completely smooth. If it still is not completely melted, microwave for another 15 seconds at a time, being careful not to scorch it.
Place the butter, sugar, salt, lemon zest, and lemon juice in the bowl of a stand mixer fitted with a paddle attachment. Mix on low speed to cream the ingredients together. Raise to medium speed and mix until the butter absorbs most of the liquid and sugar and no longer slides around (or only a little bit!).
Add the melted white chocolate and beat to incorporate.
Add the dry ingredients and mix until a dough forms. Press the dough into a 10-inch round tart pan with a removable bottom, making sure to cover the entire bottom and sides about ⅛-1/4 inch thick, discarding any leftover dough, if you have any (or bake it into a mini tart like I did). Prick the bottom all over with a fork.
Place the tart pan on a rimmed baking sheet and bake for about 20-24 minutes, or until the edges of the crust start to turn golden brown. Let cool on a wire rack.
Rhubarb Filling
Place the rhubarb in a large pot with the sugar and corn starch and cook over medium-high heat, stirring constantly, until the rhubarb has fallen apart and the puree has thickened, about 10 minutes.
Pour the hot rhubarb into a food processor or blender and add the tofu, butter, and salt. Blend until smooth.
Pour the rhubarb filling into the baked tart shell and bake until set and the center just barely jiggles, about 30-35 minutes. Let cool completely.
Candied Kumquats
In a small saucepan, bring the sugar and water to boil over medium heat, stirring to dissolve the sugar.. Reduce heat to a simmer and add the kumquats. and cook until the kumquats are translucent, about 4-5 minutes. Transfer kumquats and syrup to a jar and let cool completely.
Assemble the tart
Strain the kumquats, reserving their liquid, and arrange them in a single overlapping layer over the top of the tart.
Place the remaining white chocolate in a microwave-safe bowl and microwave for 30 seconds. Stir, then microwave for 30 seconds more. Stir until melted and completely smooth. If it still is not completely melted, microwave for another 15 seconds at a time, being careful not to scorch it.
Add the cream and a teaspoon of the kumquat syrup to the white chocolate, stirring to incorporate.
Use a spoon to drizzle the white chocolate over the tart.
Chill the tart for at least 1 hour before serving.
Notes
*I grind my own oat flour by pulsing gluten-free oats in a food processor or blender and sifting out any large pieces.**Optional: If you can't find kumquats, you can top the tart with sliced strawberries and drizzle the chocolate over the berries.Slightly Adapted from Marbled, Swirled, and Layered by Irvin Lin (shared with permission)
This nutritional information has been automatically calculated, and as such, may be incomplete or inaccurate. Please reference the specific ingredients you use for the most accurate nutritional information.