While the plum mixture is cooking, stir the almond flour, gluten-free rolled oats, oat flour, coconut flour, coconut sugar and cinnamon/nutmeg together in a mixing bowl.
Warm the coconut oil and almond butter, and stir them together. Pour the mixture into the dry ingredients, and stir until the dry ingredients are completely mixed with the almond butter and oil.
Add water by tablespoons until the dough is moistened and it sticks together but is still a little crumbly. In my kitchen, this took 3 tablespoonfuls until a handful of the crust would stick together but could be made crumbly again by chopping with a spoon.
Press ⅔ of the crust into the prepared pan, reserving the rest for the topping. Place the topping into the refrigerator.
Bake crust for 10-12 minutes and then set aside to cool.
When the crust has cooled, pour in the plum jam and spread it evenly over the baked crust.
Sprinkle the remaining crumble topping evenly over the top of the plum jam.
Bake for 25 to 30 minutes, or until the topping is a nice golden brown and the jam filling is bubbling up on the sides.
Let the crumble cool completely on the counter, then in the refrigerator before cutting into squares, at least an hour. You can store this in the refrigerator for 3-4 days or in the freezer for a few months. Be aware that the crust will get softer with time as moisture from the filling soaks into it.