Step 1: Preheat the oven to 325°F (160°C) and grease a 9-inch metal tart pan with a removable bottom. In a large bowl, whisk together the almond flour, Swerve Brown, Swerve Granular, coconut flour, cinnamon, and salt, breaking up any clumps. Stir in the melted butter until well combined.
Step 2: Press about 2/3 of the dough firmly into the bottom of the prepared pan. There's no need to go up the sides. Bake for 15–20 minutes, until just golden brown around the edges. Remove and let cool for 15 minutes to firm up.
Step 3: While the crust cools, prepare the rhubarb filling. In a medium saucepan, combine the chopped rhubarb, water, and allulose (or BochaSweet). Bring to a simmer and cook for about 5 minutes, until the rhubarb has softened and released some of its juices. Stir in the glucomannan (or xanthan gum), which will help thicken the filling. Let it cool to room temperature.
Step 4: Once the rhubarb filling has cooled, spread it evenly over the cooled crust. Add the chopped pecans or walnuts (if using) to the remaining crust mixture, then sprinkle this crumble topping evenly over the rhubarb filling. Gently press the crumble topping to adhere.
Step 5: Bake for another 20 minutes, or until the crumble topping is golden brown. Remove from the oven and let cool completely in the pan before attempting to remove the sides (this is crucial for the tart to set properly).
Step 6: Once fully cooled, gently loosen the tart from the pan using a small sharp knife if needed. Remove the sides and carefully transfer the tart to a serving plate. Slice into 10 equal portions and serve. Enjoy your delicious keto rhubarb crumble tart!