Preheat your oven to 350°F (177°C) and spray a muffin pan with nonstick cooking spray.

In a large mixing bowl, combine old-fashioned rolled oats, ground cinnamon, and any other dry ingredients. Whisk them together.

Add in your chosen wet ingredients, mixing gently until a cohesive batter forms, being careful not to overmix.

Fold in the finely chopped apples and chopped nuts, ensuring they're evenly distributed throughout the batter.

Spoon the mixture into the muffin cups, filling each about three-quarters full.

If desired, sprinkle coarse sugar on top of each unbaked cup.

Place the muffin pan in the preheated oven and bake for 28-30 minutes until the edges are golden.

Remove from the oven and let cool for 5-10 minutes before lifting the cups from the pan.