Prep the pan and oven: Heat the oven to 350°F (175°C). Line an 8x8-inch pan with parchment, leaving overhang for easy lifting.

Make the rhubarb filling: In a bowl, combine chopped rhubarb, 1/3–1/2 cup granulated keto sweetener (to taste), lemon zest, lemon juice, and cinnamon. If using a thickener, sprinkle in 1/2 teaspoon and toss well. Set aside to lightly macerate while you make the crust.

Mix the crust: In a large bowl, stir together almond flour, coconut flour, 1/4 cup sweetener, and salt. Add melted butter, egg, and 1 teaspoon vanilla. Mix until it forms a soft, slightly sticky dough.

Press and par-bake: Press about 2/3 of the dough firmly into the lined pan in an even layer. Dock lightly with a fork. Bake for 10–12 minutes until set and just starting to color at the edges.

Prep the crumble: To the remaining dough, add 1–2 tablespoons sweetener and the chopped nuts (if using). Pinch into small clumps. If the mixture feels too soft, chill it for 10 minutes.

Add the filling: Stir the rhubarb again, then spread it over the hot crust in an even layer. Include any juices; the thickener will help it set.

Top and bake: Sprinkle the crumble evenly over the rhubarb. Bake 25–30 minutes, or until the topping is golden and the filling is bubbling at the edges.

Cool completely: Let the pan cool on a rack until room temperature, then chill for at least 1 hour. This helps the bars set and slice cleanly.

Slice and serve: Use the parchment to lift the slab out. Cut into 12–16 bars. Enjoy as-is, or add a dollop of whipped cream.