Preheat the oven to 300F and line a baking sheet with parchment paper.
In a food processor, combine the almond flour, shredded coconut, chopped pecans, sweetener, cinnamon, and salt. Pulse a few times to combine. Sprinkle with the butter pieces and then process until the mixture resembles coarse crumbs.
Spread the mixture on the prepared baking sheet and cover with another piece of parchment. Use a rolling pin or your hands to press down firmly so that the mixture sticks together. Remove the top sheet of paper.
Bake 20 to 25 minutes, or until edges are golden brown. Remove and let cool completely - the mixture will continue to crisp up as it cools.
Preheat the oven to 350F. Spread the rhubarb in a 9x13 inch glass or ceramic baking dish and sprinkle with the sweetener and glucomannan. Toss to combine.
Cover tightly with foil and bake 30 to 35 minutes, or until the mixture is bubbling and rhubarb is tender.
Remove the filling from oven and remove foil. Break the cooled topping into pieces with your hand and place over filling to cover completely. Serve warm, topped with whipped cream or keto vanilla ice cream.