In a large bowl, combine all the ingredients for the fruit. Toss to combine and set aside.
Make the topping:
Combine the oats, walnuts, sugar, arrowroot, and salt. Add the coconut oil or butter. Use your fingers to work the oil/butter into the oat mixture until you have a crumbly topping.
Assemble:
Give the fruit mixture another toss, then transfer to a 9-inch pie dish or 8x8-inch baking dish. Crumble the topping over the top and press gently to adhere. Place the dish on a rimmed baking sheet and transfer to the oven. Bake for 45-55 minutes or until deeply golden brown and bubbly. Cool for at least 1 hour before serving. (Store leftovers in the refrigerator.)