Start by trimming the rhubarb at either end and the chop it. I do an even mix of finely diced—halving width-wise and then in half for thin stalks or in thirds for thicker ones—and rough, larger bits. Toss the rhubarb with the sugar and salt and set it aside to macerate for 30 minutes.
Preheat over to 375°. Spray a 9" x 13" pan with baking spray and lay a fitted piece of parchment on the bottom.
Next start the crust. Add the flour, almond flour, sugar, baking powder, and sugar in a food processor. Tip in the cold, diced butter and pulse about 10 times to break the butter up into bits around the size of peas.
Slightly beat the egg in a separate dish and quickly add it into the processor while the motor is running. Immediately shut it off. Mix it a little more by hand to make sure the egg is evening incorporated throughout. The dough will be incredibly crumbly. Press half of the dough firmly into the bottom of the prepared pan.
Drain the liquid from the rhubarb—if you don't the bars will be soggy (don't toss it! Add it to soda water, reduce to a syrup... you get the idea). Toss the rhubarb with the cornstarch. Scatter this across the crust.
Sprinkle the remaining half for the crumbly mix over the top, squeezing and clumping some together as you do so, similar to how you would make a crumble. Bake for 45 - 55 minutes, until the crumbly topping is golden brown and the rhubarb soft beneath.
Let the bars cool to room temperature. I think they're easier to cut after a quick trip in the fridge to chill them. These are best within a day or two of baking but I'm on day 4 and have no complaints.