1. Pre-heat oven to 350º. Grease 9 x 13 inch baking dish.
2. In a large bowl, combine the almond flour, sugar substitute, baking powder, and salt.
3. Add the butter and use a fork or pastry blender to cut the butter.
4. Mix until the pieces are pea sized.
5. Add the beaten egg and continue to mix until well combined.
6. Divide the crumble mixture and press half into the bottom of baking dish.
7. In another medium sized bowl, mix together the strawberries, rhubarb, xanthin gum and sugar substitute. Add this mixture to the the top of the crumb base, making sure to spread evenly.
8. Top the bars with the other half of the crumb mixture.
9. Bake until the top is golden brown and fruit is tender for about 40 minutes.
10. Serve warm as a crumble or allow to cool completely to slice and serve as bars.