Preheat oven to 375 degrees. Grease a 2 quart baking dish (or 8 inch square baking pan) and set aside.
In a large bowl, add all filling ingredients: blueberries, cornstarch, sugar, lemon juice, lemon zest, and vanilla extract. Toss to combine. Pour filling into prepared dish and set aside.
In a separate bowl, add dry ingredients for your topping: flour, oats, cinnamon, brown sugar, and salt. Add melted butter and stir until combined. Mixture will be thick and crumbly.
Crumble topping evenly over blueberry filling. Bake for 55-60 minutes, or until blueberry filling bubbles in the middle of the dish.
Allow to cool slightly before serving. Serve warm alone or with a scoop of vanilla ice cream.
Notes
Use a 2 quart baking dish or 8x8 square baking pan for best results.
Frozen blueberries can be used. Thaw before baking, or add up to ten minutes of additional baking time if frozen.
Blueberry crisp will keep for 3 days, covered, at room temperature, or up to a week in the refrigerator.