Grease six mini cast iron skillets. For the crumble topping, whisk together oats, flour, brown sugar, cinnamon, and salt in a medium bowl. Cut in butter with a pastry blender or fork until large crumbs form. Keep chilled while preparing filling.
For the filling, whisk together sugar, arrowroot starch, and salt in a medium bowl. Add blueberries and lemon juice; toss to combine. Divide filling evenly among skillets (about 1/2 cup per skillet) and sprinkle with chilled topping, pressing gently to adhere. Chill assembled crumbles for at least 1 hour in the refrigerator, or up to 1 day.
Preheat oven to 375°F. Bake crumbles for 30-35 minutes, until filling is bubbling and topping is golden brown. Cool for at least 30 minutes.