Peel apple, remove the core, slice thinly then sliced in half. The apple slices need to be small to fit in the jars.
In a large bowl, toss apples with lemon juice to prevent browning and add flavour. Add blackberries.
Sprinkle with cornstarch or tapioca starch, sweetener, cinnamon, nutmeg and salt.
Melt butter and combine the crumble topping ingredients.
Fill 12 half-pint Mason jars with apple-blackberry mixture, almost to the top. Overfill with crumble topping. Once the jars come out of the oven, the filling will have cooked down and there will be space at the top of the jar.
Place the jars on a baking sheet in case the filling bubbles over. Bake for 25 - 30 minutes or until fruit is bubbling and crisp topping is golden brown.
Cool slightly and serve warm or cold, plain or with whipped cream, whipped coconut milk or ice cream.
Cover jars with lids and refrigerate leftovers, or freeze.
No Mason Jars? Bake this apple blackberry crisp in a 9-inch round baking dish or a 9 x 13 baking dish.