1. Preheat your oven to 375°F and prepare your baking dish by lightly greasing it or lining it with parchment paper.
2. In a large bowl, combine 4 cups of fresh or frozen blueberries with your chosen sweetener. If using frozen blueberries, add a tablespoon of cornstarch to thicken the filling during baking. Optionally, enhance the flavor with a teaspoon of lemon juice and a sprinkle of lemon rind.
3. Prepare the crumble topping by mixing oats, flour, sugar, cinnamon, and a pinch of salt in a separate bowl. Add butter or coconut oil and mix until the texture resembles coarse crumbs, either using your fingers or a pastry cutter.
4. Evenly spread the blueberry mixture across the bottom of the prepared baking dish. Generously sprinkle the crumble topping over the blueberries to cover them completely.
5. Place the baking dish in the preheated oven and bake for 25-45 minutes, checking during the final 10 minutes to ensure the top doesn’t over-brown. The crisp is ready when the blueberries are bubbling, and the topping turns golden brown.
6. Remove the blueberry crisp from the oven and let it cool slightly, allowing the filling to thicken for easier serving.
7. Serve the warm blueberry crisp on its own or pair it with a scoop of vanilla ice cream for an elevated dessert experience.