In a large bowl, combine all filling ingredients (blueberries, sugar, cornstarch, lemon juice, lemon zest and vanilla). Mix well. Add mixture to an 8x8 baking dish and set aside.
Combine flour, oats, brown sugar, salt, cinnamon, and ground ginger in a bowl and mix well. Add to mixture softened butter and work it into the flour mixture with a fork (or by hand) until well combined.
Add flour mixture to the top of the blueberry mixture and spread it out evenly so it covers the blueberry filling.
Bake for 40-45 minutes until mixture is bubbly and topping is slightly brown. Allow mixture to cool for at least 10 minutes before serving.
If using frozen blueberries, be sure to defrost and drain them to avoid having a watery crisp.Adjust sugar to your liking. For example, use 1/4 cup brown sugar instead of 1/2 brown sugar for a less sweet, more tart crisp.The crisp tastes just as good on day two. Store leftovers in the fridge and enjoy cold or reheated the next day (as if it will last that long).