Step 1.  Pre-heat oven to 375 degrees F and liberally grease a 9-inch springform pan.  

Step 2.  Begin with the coffee cake ingredients.  In a large bowl, with an electric mixer, cream together 6 tablespoons butter and 1/4 cup sweetener until light and fluffy.

Step 3.  Beat in eggs, one at a time, and then add vanilla.

Step 4.  In a medium-sized bowl, mix together salt, almond flour, coconut flour, baking powder, and xanthan gum.  Beat the dry ingredients into the wet.  Then beat in water and cream.  

Step 5.  The batter will be very thick.  Pour batter into the springform pan and smooth the top with a spatula.

Step 6.  Bake for 18-24 minutes until top is browned and a toothpick in the middle comes out clean.

Step 7.  As soon as the cake goes in the oven, you can make the rhubarb sauce.  Add rhubarb, water and sweetener to a medium-sized saucepan and bring to a boil over Medium-High heat. 

Step 8.  Simmer until rhubarb is soft and partially broken down and the liquid has reduced 4 minutes.  Taste the rhubarb filing for sweetness and add more sweetener if desired.   Sprinkle xanthan gum over the rhubarb and quickly stir it in.  Simmer for 2 minutes more until thickened.  If it's not a pretty pink color, you can add a few drops of red food coloring, if you like.  

Step 9.  For the streusel topping, in a medium-sized bowl, mix together 1/4 cup sweetener, 3/4 cup almond flour, and 1/3 cup sliced almonds.  Stir in melted butter until crumbs form.

Step 10.  Spoon the rhubarb over the cooked coffee cake and smooth it from edge to edge.  Crumble streusel on top.

Step 11.  Turn the heat up to broil, but leave the rack in the middle.  Broil for 1-2 minutes until crumble topping is browned.  Keep a very close eye on it because it will brown quickly.

Step 12.  Cool completely in the pan.  Carefully remove the outer wall of the springform pan and cut into 10 slices.