Preheat your oven to 425°F and line a 12-cup muffin tin with paper liners.
Prepare the streusel by combining a portion of the flour, oats, cinnamon, and brown sugar in a bowl.
Cut the cold butter into the streusel mixture until pea-sized crumbs form, then refrigerate until needed.
In a large bowl, whisk together the dry ingredients including both flours, baking powder, baking soda, and salt.
In a separate bowl, combine Greek yogurt, maple syrup, egg, and the remaining brown sugar until smooth.
Fold the dry ingredients into the wet mixture until just combined, then gently stir in the diced apples.
Distribute the batter evenly into the prepared muffin tin.
Top each muffin generously with the chilled streusel mixture.
Bake at 425°F for 5 minutes, then reduce the heat to 350°F and bake for an additional 18 minutes until golden and set.
Cool in the pan for 5 minutes before transferring to a wire rack.