Preheat the oven to 325F and grease a 9-inch metal tart pan with a removeable bottom.
In a large bowl, whisk together the almond flour, sweeteners, coconut flour, cinnamon, and salt, breaking up any clumps with the back of a fork.
Stir in the butter until well combined. Press about 2/3 of the dough firmly and evenly into the bottom of the prepared pan. No need to go up the sides.
Bake 15 to 20 minutes, until just golden brown around the edges. Then remove and let cool at least 15 minutes to firm up.
Filling
While the bottom crust is baking, prepare the rhubarb filling.
In a medium saucepan over medium heat, combine the rhubarb, water, and allulose. Bring to a simmer and cook 5 minutes, until softened and the rhubarb releases some juices.
Stir in the glucomannan and let cool to room temperature.
To assemble
Once the filling is cool, spread evenly over the cooled bottom crust.
Add the chopped nuts to the remaining crust mixture. Sprinkle the crust mixture over the filling, pressing lightly to adhere.
Bake another 20 minutes, until the top crust is beginning to brown. Remove and let cool completly in the pan.
Use a small sharp knife to gently loosen the side of the pan and remove the sides before cutting into slices.