Preheat oven to 375° F. Spray a muffin pan with cooking spray.
In a medium bowl, stir together the Greek yogurt, milk and oats. Set aside to soak for 10-15 minutes.
¾ cup (170g) plain Greek yogurt, ½ cup (120mL) whole milk, 1 cup (85g) old-fashioned rolled oats
Meanwhile, in a large mixing bowl, combine the melted butter, brown sugar, eggs and vanilla. Whisk until well combined.
6 tablespoons (85g) unsalted butter, ½ cup (100g) packed light brown sugar, 2 large eggs, 1 teaspoon vanilla extract
Add the cinnamon, baking powder, baking soda, salt and nutmeg. Whisk until very well combined.
1½ teaspoons ground cinnamon, 1 teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt, ¼ teaspoon ground nutmeg
Add the oat mixture and use a rubber spatula to mix it into the other ingredients.
Add the flour. Use a rubber spatula to mix until almost combined. Be careful to not overmix.
1 ½ cups (180g) all-purpose flour*
Gently fold in the diced apple until just combined.
1 ½ cups (160g) diced fresh apple*
Scoop the batter into the prepared muffin pan, dividing it evenly between the 12 wells. Bake muffins for 17-19 minutes, or until a toothpick inserted into the center of a muffin comes out clean. (Bake mini muffins for 11-13 minutes.)
Let cool in the pan for 10 minutes and then transfer to a wire rack to finish cooling. Serve warm or at room temperature.