Preheat the oven to 350 degrees F (175 degrees C). Grease 2 muffin tins or line with paper liners.
Combine flour, baking soda, and flax seeds in a bowl; make a well in the center.
Place spinach, applesauce, avocado, apple, milk, apple, sugar, canola oil, eggs, brown sugar, and vanilla extract in a blender or food processor; puree until smooth.
Pour the spinach puree into the well of the flour mixture and mix well. Scoop batter into the prepared muffin tins.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 20 to 22 minutes.
