Preheat the oven to 350°F. Lightly mist a 9x13-inch glass, ceramic, or nonstick baking pan with nonstick oil spray (with this much lemon juice, avoid uncoated aluminum pans, which can lend a metallic taste). Line the pan with a large piece of parchment paper that hangs over the two long (13-inch) sides—this “sling” is how you'll lift the finished bars out of the pan. Mist the parchment and the two exposed short ends of the pan, too.

Make the crust: In a large bowl, stir together the almond flour, flaxseeds, and salt with a fork. Pour in the melted coconut oil and maple syrup and stir until evenly combined. Add the almond milk 1 tablespoon at a time, stirring after each addition, just until the dough holds together when you squeeze a handful in your palm—you may not need all 8 tablespoons. (Too dry and it will crumble; too wet and it turns pasty, so go slowly toward the end.)

Transfer the dough to the lined pan and press it firmly into an even layer across the bottom—the flat bottom of a measuring cup or drinking glass makes quick, even work of this. Then press the dough about ½ inch up all four sides of the pan to create a low “wall.” Don't skip this! That little rim is what keeps the liquidy filling contained on top of the crust instead of seeping underneath it.

Poke the dough all over with a fork (this lets steam escape so the crust bakes flat) and bake for 18 to 20 minutes, until the edges are lightly golden and the surface looks dry.

While the crust bakes, make the filling: In a small bowl, whisk the eggs and egg white until smooth; set aside. In a large bowl, whisk the arrowroot flour with a few tablespoons of the lemon juice to make a smooth paste, then gradually whisk in the rest of the juice until completely smooth and lump-free. Whisk in the maple syrup, lemon zest, and salt, then the eggs, whisking gently until everything is fully combined—no need to beat air into it (fewer bubbles = a smoother, glassier top). If a foamy layer forms, skim it off with a spoon.

Give the filling one last stir right before pouring (arrowroot settles fast). For the steadiest pour, pull the oven rack partway out with the hot crust still on it, then slowly pour the filling into the center of the crust and carefully slide the rack back in. Bake for 22 to 28 minutes, until the edges are set and the center still has a slight Jell-O-like jiggle when you gently nudge the pan. This is the key moment: the filling will continue to firm up as it cools, so don't wait for the center to look fully set in the oven—over-baking is what causes cracks and a weepy texture.

Cool the pan on a wire rack until it reaches room temperature, about 1 hour, then refrigerate (loosely covered once cool) for at least 3 hours, or overnight, until fully chilled and set.

When you're ready to serve, run a thin knife along the two unlined short ends of the pan, then lift the bars out using the parchment sling and set them on a cutting board. Slice into 24 squares (6 rows by 4 rows), wiping the knife clean with a damp paper towel between cuts for the prettiest edges. Dust with an optional sprinkle of confectioners' sugar just before serving.