Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease it with butter.
In a medium bowl, combine rolled oats, ½ cup all-purpose flour, ¼ cup brown sugar, and ½ teaspoon ground cinnamon. Add cold butter and mix with your fingers or a pastry cutter until the mixture resembles coarse crumbs. Set aside.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, ground cinnamon, and nutmeg.
In another bowl, beat together the granulated sugar, brown sugar, eggs, vegetable oil, milk, pumpkin puree, and vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients and gently fold until just combined. Fold in the diced apples.
Using a spoon or an ice cream scoop, fill each muffin cup about ¾ full with the batter.
Generously sprinkle the streusel topping over each muffin.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Remove from oven and allow the muffins to cool in the pan for about 5 minutes before transferring them to a wire rack to cool completely.
Notes
Serve the muffins warm, dusted with powdered sugar, and accompanied by a small bowl of whipped cream or a side of apple slices for an extra touch!