Place a rack in the center of the oven and preheat to 375 degrees. Spray a 2 qt baking dish or an 8" square pan with non-stick spray and set aside.
In a medium-sized mixing bowl, mix the blueberries, sugar, cornstarch, lemon zest and lemon juice, vanilla and salt until well combined. Transfer the berry mixture into the prepared baking dish. NOTE: When working with fresh fruit, the amount of thickener you need depends on how ripe the fruit is. If you’re using super sweet, ripe fruit, increase the cornstarch by 1 or 2 Tbsp.
Wipe out the mixing bowl and add the oats, flour, brown sugar and cinnamon and stir to combine. Add the butter and mix until the butter chunks are the size of peas and the mixture becomes crumbly. It is easiest to form the crumble by pinching the ingredients between your fingertips, but if you have a stand mixer, use low speed and pulse until the crumble forms.
Sprinkle the crumble mixture on top of the blueberries. Do not press or compact the crumble. Bake 30−35 minutes until the crumble is nicely browned and the juices are bubbly.
Serve warm with vanilla ice cream or whipped cream. Enjoy!
Notes
If you choose to use frozen berries, just defrost them before beginning the recipe and double the amount of cornstarch—keep everything else the same.