Combine flour, oats, cinnamon and brown sugar in a medium bowl. Cut in the butter until crumbs form. Chill.
Make the muffins:
Preheat your oven to 425°F. Line a 12 cup muffin tin with paper liners.
Mix all dry ingredients in a large bowl.
Whisk together all wet ingredients in a large measuring jug.
Fold the wet into the dry ingredients just until combined, do not overmix (a few lumps are fine, but there shouldn't be any obvious flecks of dry flour). The batter will be fairly thick. Fold in the apple chunks.
Evenly divide the batter between the prepared muffin cups (this is really easy and mess free with an ice cream scoop). Sprinkle with the chilled streusel.
Reduce the oven temperature to 400°F. Bake the muffins for 5 minutes. Turn down the oven temperature to 350°F and finish baking the muffins for 15-18 minutes or until a toothpick inserted into the middle comes out clean. Cool on a wire rack for 5 minutes before removing from the muffin pan and cooling on the rack completely.
I topped the muffins with a quick glaze to make them prettier in the photos. If you want to glaze yours, I simply stirred 1-2 teaspoons of maple syrup into a couple of tablespoons powdered sugar until smooth.
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