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1
Start by making the almond cream, as it has to chill for the longest amount of time. Finely ground the almonds and flour in a food processor. Add the sugar and process again. Then add the butter, extract, and lemon zest and blend until smooth. Mix in egg and egg white and process until no lumps remain. Transfer filling to a medium bowl, cover, and chill for at least 3 hours. (You can also make this 2 days in advance.)
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2
To make the tart crust, pulse the flours, sugar, and salt together in a food processor. Scatter the butter pieces over top and pulse until it's coarsely cut in-the size of oatmeal flakes and peas.
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3
Add the egg a little at a time, pulsing after each addition. When the egg is all in there, process in long 10-second pulses until the dough forms clumps and curds (this typically takes me 5 to 8 of these long pulses). The sound of the machine will change just before you get to this point, so listen carefully!
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4
Turn the dough out onto a piece of plastic wrap spread on a work surface. Very gently, bring all of the disparate pieces together. Blanket with plastic wrap and flatten into a disc. Chill the dough in the refrigerator for 2 hours (if you chill it for more time than this, you'll need to let it soften just a little at room temperature before rolling it out).
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5
Butter a 9-inch fluted tart pan with a removable bottom. On a floured sheet of parchment paper, roll the dough out to a 12-inch round, occasionally lifting it up and sprinkling some flour underneath. (You can also roll the dough out between 2 pieces of lightly-floured plastic wrap.)
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6
Use the paper as a guide to help you lift the dough into the tart pan. Peel off the paper, then gently lift and nudge the dough into the sides of the pan. Reserve any extra dough (for repairing cracks, for extra insurance!) in the refrigerator. Cover with plastic wrap and freeze for at least 30 minutes (and preferably longer).
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7
While your crust freezes, make the rhubarb jam. Trim the ends off the rhubarb stalks and then cut any large ones in half lengthwise, Cut the lengths into 3/4-inch chunks. You should have about 6 cups of rhubarb; set 2 cups aside.
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8
Add the remaining 4 cups of rhubarb to a medium sauce pan with the brown sugar and hibiscus flowers. Stir so that everything is combined, then turn the heat to medium low. Cover the pot and cook for 15 minutes, stirring from time to time just to make sure the sugar isn't burning.
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9
Remove the cover, increase the heat to medium, and cook for around 15 minutes, stirring constantly, until the rhubarb has completely broken down and the consistency is thick.
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10
Add the remaining rhubarb to the pot and stir to combine.
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11
Pour the compote into a large baking dish or a sheet tray to cool. Once cool, remove the hibiscus flowers.
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12
While the jam cools, par-bake your tart crust. Preheat the oven to 375u00b0 F and position a rack in the center. Butter a piece of nonstick aluminum foil and place it butter side down tightly against the dough. Place on a baking sheet and bake on center rack for 20 to 25 minutes.
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13
Carefully remove the foil. If the crust is puffy, gently press it down with the back of a spoon. Bake for 5 additional minutes, then transfer to a rack and cool to room temperature.
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14
When your tart is partially baked, the rhubarb jam is cool, and the almond cream has chilled for 3 hours, you're ready to assemble and do the final bake!
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15
Preheat the oven to 350u00b0 F with a rack in the center. Spread the compote in a rather thin layer over the base of the shell. Save any extra throughout the week in the fridge. Then dollop the almond filling over top and spread it carefully with an offset spatula. If using, sprinkle the hibiscus sugar over top. Bake tart until golden and tester inserted into center of filling comes out clean, about 45 minutes. Cool tart in pan on rack. The whole tart can also be made half a day in advance. Let stand at room temperature.
