In a small bowl, combine the orange juice, arrowroot powder, and maple syrup.
In a large bowl, combine the strawberries and rhubarb. Add the orange juice mixture and stir. Pour the fruit into a 9x12-inch baking dish that has been coated with cooking spray.
Add the rolled oats, almond flour, sugar, cinnamon, salt, and butter to a food processor. Pulse until the mixture is crumbly. Transfer this to a bowl and add the walnuts.
Top the fruit with the oat topping. Bake for 40 minutes and let cool 30 minutes before serving.
Serve the crisp with ice cream or whipped cream, if desired.