Preheat oven to 400°. Grease one 12 cup muffin tin or one 6 cup jumbo muffin tin. Set aside.
In a large mixing bowl, combine egg, milk, oil, applesauce Greek yogurt, honey, and brown sugar. Stir.
In a separate mixing bowl, combine flour, oats, flax, chia, baking powder, and salt. Add to wet ingredients and mix just until all ingredients are combined.
Gently fold in chopped apples and blueberries.
Spoon into prepared muffin tins, filling each cup 3/4 full.
Combine topping ingredients; divide evenly among muffins. Press topping down gently.
Bake at 400° 20 minutes for 12 muffins or 25 minutes for 6 muffins.
Remove from oven; let cool 15 minutes in pan. Remove to wire rack to cool completely.