Preheat and prep: Preheat your oven to 350°F (175°C). Grab an 8x8-inch baking dish and lightly grease it. This is where the magic happens.
Mix the berries: In a medium bowl, toss the blueberries with sugar, lemon juice, vanilla, and cornstarch. Make sure every berry is coated—think of it like a blueberry spa day.
Transfer to dish: Pour the berry mixture into the prepared baking dish, spreading evenly.
Make the topping: In a separate bowl, mix oats, flour, brown sugar, salt, and cinnamon. Pour in the melted butter and stir until crumbly. Yes, it should feel like sand but in the best way.
Top the berries: Sprinkle the oat mixture evenly over the blueberry filling. Don’t skimp—it’s the best part.
Bake: Slide the dish into the oven and bake for 35–40 minutes, or until the topping is golden brown and the berries are bubbly.
Cool slightly: Let it sit for 5–10 minutes before digging in. FYI, it’s super tempting to eat it straight from the oven, but patience will be rewarded.