Begin by placing the blackberries in a mixing bowl, then sprinkle with sugar and squeeze in the lemon juice. Gently toss to coat the berries evenly and let sit for 10 minutes, allowing the juices to start seeping out and brightening the flavor.
Preheat your oven to 350°F (175°C) and butter your baking dish generously to prevent sticking and ensure easy cleanup.
In a separate bowl, combine the oats, flour, cinnamon (if using), and a pinch of salt. Mix well to distribute all ingredients evenly.
Add the melted butter to the dry mixture and use your fingers or a fork to rub it in, creating a crumbly, coarse texture that resembles wet sand.
Pour the sugared berries into the prepared baking dish, spreading them out evenly so they cover the bottom in a single layer.
Sprinkle the crumbly oat topping evenly over the berries, covering them completely and gently pressing it down to adhere.
Place the dish in the preheated oven and bake for about 30 minutes, or until the topping turns golden brown and the filling bubbles around the edges with fragrant juices.
Remove the crisp from the oven and let it rest for a few minutes. The topping will crisp up further as it cools slightly, and the juices will thicken a bit.
Serve your blackberry crisp warm, ideally with a dollop of whipped cream or a scoop of vanilla ice cream for extra indulgence.
