How to Make It
Make the muffins. Preheat the oven to 375°F. Place silicone or paper liners in 15 cups in muffin pans or coat the cups with cooking spray.
In a medium bowl, mix together the flours, flaxseed, baking powder, baking soda, and salt.
Spoon the avocado flesh into the bowl of a stand mixer with a beater attachment. Beat on low until almost smooth. (If you don’t have a stand mixer, simply mash up the avocado as much as possible with a fork.) With the mixer still running on low, add the sugar. Once the sugar is mixed in, beat in the egg. Add the vanilla and yogurt and mix well.
Slowly add the flour mixture to the wet ingredients and mix until just blended, being sure not to overmix. At this point, the batter will be a bit thick. Using a spoon, gently fold in the blueberries.
Fill the muffin cups about three-fourths full, leaving room for the streusel topping.
Make the streusel topping. In a medium bowl, combine the flour, sugar, and lemon zest. Using a pastry cutter or a fork, cut in the butter until it resembles coarse crumbles.
Sprinkle the streusel topping evenly over the batter.
Bake for 25 to 30 minutes, or until a toothpick inserted in the center of a muffin comes out clean. Let the muffins cool in the pan on a rack for 5 to 10 minutes before removing.
Serve warm or at room temperature.
Freeze the muffins. After the muffins have cooled, place them in an airtight freezer container or freezer bag. To prepare muffins from frozen, wrap a frozen muffin in a moist paper towel and microwave in 30- to 60-second increments until warmed all the way through.