Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease with cooking spray.
In a large mixing bowl, combine the rolled oats, baking powder, cinnamon, and salt. Mix well to ensure that the dry ingredients are evenly distributed.
In another bowl, whisk together the apple sauce, honey (or maple syrup), egg, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients, stirring until just combined. Fold in the diced apples and nuts (if using), and add the raisins or cranberries (if desired).
Divide the oat mixture evenly among the muffin cups, filling each cup about 3/4 full.
Bake in the preheated oven for 25-30 minutes, or until the tops are golden and the centers are set. A toothpick inserted in the center should come out clean.
Allow the oatmeal cups to cool in the pan for about 5 minutes, then transfer them to a wire rack to cool completely.
Notes
Serve warm, drizzled with honey and garnished with cinnamon or a slice of fresh apple.