In a large bowl, combine the blueberries, sugar, cornstarch, lemon juice, lemon zest, vanilla, and a pinch of salt. Mix well, then add the mixture to a 7x7 round baking dish or a dish of a similar size.
In another large bowl, combine the flour, oats, brown sugar, salt, cinnamon, and ground ginger and mix well. Add the cold butter pieces and work the butter in with a fork, then use your hands. The mixture will be crumbly. Add the mixture to the top of the blueberry mixture.
Bake the crisp for 40-45 minutes. The blueberries will be bubbly. Allow the mixture to cool and set for at least 15 minutes before serving.
Notes
If using frozen blueberries, be sure to defrost and drain them to avoid having a watery crisp.If you're using a larger baking dish, consider adding more fresh blueberries (1 cup). You don't need to adjust the recipe much otherwise.This is a small batch that feeds my family, but you can of course double the recipe to feed a crowd.Store leftovers covered with plastic wrap in your fridge. Enjoy cold or microwave it for a few seconds to warm it up. The leftover crisp tasted good for several days.