Preheat oven to 325 degrees. Grease a 9″x13″ or other large casserole dish.
Place cassava flour, coconut flour, coconut sugar, and sea salt into food processor. Pulse briefly to combine. Add butter and pulse repeatedly to combine, until largest butter pieces are no bigger than pea size.
Add egg yolks and pulse again until meal begins to clump in places. Set aside while you assemble the rhubarb.
In casserole dish combine rhubarb, water, sweetener, and gelatin, tossing to combine with two large spoons. Dot with small pieces of the butter or preferred fat.
Bake rhubarb alone first, without the topping, for 25 minutes in preheated oven.
Remove casserole dish from oven; add topping: Use your hands to squeeze small clumps of the dough together. Cover the surface of the rhubarb with both clumps and small crumbles of dough.
Bake crumble an additional 25 minutes, until the sides are very bubbly, the center is very hot and the topping is golden brown.