Preheat the oven to 190°C (375°F) and set a 23 cm (9-inch) baking dish aside.
In a large mixing bowl, combine the blackberries, honey, starch, and vanilla. Stir to evenly coat the berries. Transfer to the baking dish and set aside.
To make the oat topping, mix the oats, sugar, flour, nutmeg, and salt in a bowl to combine. Add the oil, milk, and vanilla extract. Use your hands to mix until it resembles large crumbs.
Sprinkle the topping in an even layer over the blackberries.
Place the baking dish onto a baking sheet and bake for 40 to 45 minutes, or until the topping is lightly golden and the fruit juices are bubbling around the edges of the dish.
Remove from the oven and cool on a wire rack for at least 15 minutes before serving. Blackberry crisp freezes well and can be stored in the fridge for up to 3 days.
Notes
Storage: refrigerate in a sealed container for up to three days. The topping softens as it sits but you can reheat in the oven to crisp again.Freezing: transfer the fully cooled crisp to an airtight container and freeze for up to three months. Thaw at room temperature and reheat in the oven to re-crisp the topping.