Preheat the oven to 375 degrees Fahrenheit.
Make topping: In a medium bowl, mix together the oats, flour, brown sugar, cinnamon, salt, and butter. Set aside.
Mix the filling: In another medium bowl, mix together the strawberries, rhubarb, lemon juice, brown sugar, and arrowroot powder.
Pour the strawberry rhubarb filling into a 9x9 baking pan. Spread evenly with a spatula or spoon. Top with the oat crumble topping, spreading evenly.
Bake for 45 minutes or until the fruit is tender and bubbling. Serve with ice cream or coconut whipped cream if desired.