Preheat the oven to 350ºF. Using a stand mixer or hand-held mixer, cream the butter, lemon zest & maple syrup on high for 1 minute. If the mixture is clumpy (often due to maple syrup being cold), don't worry, it will still turn out!
Add the egg and vanilla and beat until combined.
Toss the blueberries with ¼ cup of flour in a bowl, set aside.
Whisk together the remaining flour, baking powder and salt in another bowl. Add half of the flour mixture to the wet batter and stir gently to combine.
Next, add the milk to the batter and stir. Add the remaining flour mixture and stir until flour is absorbed. Gently fold in the flour-coated blueberries. (If there's any excess flour in bowl, leave it.)
Combine the top crumble ingredients in a small bowl, mix with a fork until mixture gets crumbly.
Grease baking pan with butter or avocado oil spray. Spread the batter into the pan. Sprinkle crumble evenly on top. Bake for 35 to 45 minutes depending on pan size. (An 8-inch pan usually needs 40 to 45 minutes, a 9-inch pan may be a little less. Check for doneness with a toothpick. Bake as long as necessary until toothpick comes out clean. Let cool for at least 15 minutes before serving.