Preheat the oven to 350 degrees. Line a standard muffin pan with paper liners or spray the cavities with cooking spray. Set aside.

Whisk together the 1 1/4 cups flour, 1/2 cup oats, baking powder, cinnamon, baking soda, and salt.

In a large bowl, vigorously whisk the 3/4 cup sugar, oil, and eggs until frothy, about 30 seconds. Whisk in the Greek yogurt and milk until incorporated.

Add the dry ingredients to the wet ingredients, stirring just until combined, then fold in the apple chunks. Divide the batter evenly between each of the 12 muffin cups.

Place the 1/2 cup flour, 1/4 cup oats, 1/4 cup brown sugar and cinnamon in a small bowl and whisk to combine. Add the cold butter chunks and incorporate into the dry mixture using your fingers, a pastry blender, or two forks until the the mixture resembles pea-sized crumbles. Top the batter with the streusel — you may not need all of it.

Bake on the middle rack of the oven for 18-20 minutes or until the tops of the muffins spring back when pressed gently, or a toothpick inserted into the center comes out with moist crumbs. Cool on a wire rack for 10 minutes, then remove from the pan and cool completely.

Place the powdered sugar in a small bowl. Add 1 teaspoon of milk and stir. Add another 1-2 teaspoons of milk depending on the desired consistency. Spoon a bit of glaze over each muffin and allow to set at room temperature. Enjoy!