Macerate the rhubarb: Toss the rhubarb, chopped with the 3 tablespoons sugar in a bowl. Let it sit at room temperature for about an hour — this macerates the rhubarb, drawing out excess liquid and turning it jammy instead of tart and stringy. Don't skip this step.
Make the flax eggs: In a small bowl, whisk together the 3 tablespoons flaxseed meal and 9 tablespoons water. Let sit about 5 minutes or until it thickens into a gel — this is your egg substitute.
Preheat your oven to 350°F. Grease a 9x9 baking dish or line it with parchment paper.
In a large bowl, whisk together the all-purpose flour, almond flour, sugar, baking powder, baking soda, and salt.
Add the thickened flax eggs and non-fat or low-fat Greek yogurt to the dry ingredients. Mix until just combined — don't overmix, a few lumps are fine.
Pour the batter into your prepared 9x9 pan and spread into an even layer.
Drain the macerated rhubarb (discard the liquid) and pile it evenly over the batter.
Bake at 350°F for 45-50 minutes, until the top is golden and a toothpick inserted into the center comes out clean.
Let the cake cool slightly, then dust with powdered sugar. Try not to eat it straight from the pan — no promises.