Place the oven rack in the center of the oven. Preheat the oven to 350°F.
Coat four small ramekins with cooking spray.
In a large bowl, add the frozen mixed berries, sugar, and instant tapioca. Stir until the berries are coated.
Divide the berry mixture evenly among the ramekins.
In the same large bowl, add the rolled oats, flour, chopped walnuts, brown sugar, walnut oil, maple syrup, salt, and cinnamon. Stir until the mixture becomes crumbly, like coarse meal.
Divide the crumb topping evenly over the berry mixture in each ramekin.
Place the ramekins in the oven.
Bake for about 30 minutes, or until the tops are browned and the berry filling is bubbling. Remove the ramekins from the oven.
Let the crumbles cool for 5 minutes before serving.
Notes
Tips and Tricks
Use the right oats - Rolled oats work best for the topping. Quick oats can turn too soft, and steel-cut oats will stay too firm.Keep the berries frozen - Add the berries while they are still frozen. This helps the filling bake properly without turning watery too soon.Place ramekins on a baking sheet - A baking sheet makes the ramekins easier to move in and out of the oven. It also catches any bubbling berry filling.Let them rest before serving - The filling will be hot when it comes out of the oven. A short rest helps it thicken slightly.
Storing and Reheating
Store in Refrigerator - Cover the cooled ramekins with plastic wrap or transfer the crumbles to an airtight container. Refrigerate for up to 3 days.Reheat in Oven - Place the ramekin on a baking sheet and warm at 325°F until heated through.Reheat in Microwave - Heat one serving in short intervals until warm. The topping will be softer than when reheated in the oven.