Preheat the oven to 350ºF. In an 8-inch square baking dish, combine the rhubarb, strawberries, coconut sugar, arrowroot, and vanilla. Toss well to coat.
In a separate bowl, combine the oat flour, coconut sugar, oats, oil, and salt. Stir well to combine, until the mixture looks evenly crumbly. Sprinkle it evenly over the fruit.
Bake at 350ºF for 45 minutes, so that the fruit is tender and the top is golden. Let the crisp cool for 15 minutes, then serve warm with ice cream on top, if desired.
Leftover crisp can be covered and stored in the fridge for up to 5 days.
You can probably swap the oat flour for regular or whole wheat flour, depending on what you have in your home. For more alternative flour ideas, like almond and coconut flour, be sure to read the post above!