Preheat the oven to 350°F (175°C) and line a muffin tin with paper liners or grease it lightly.

In a large bowl, whisk together 1 ½ cups of flour, baking powder, baking soda, salt, cinnamon, nutmeg, and ginger.

In another bowl, mix the pumpkin puree, granulated sugar, brown sugar, vegetable oil, eggs, and vanilla extract until well combined.

Gradually add the wet ingredients to the dry ingredients, stirring just until incorporated. Do not overmix. If using, fold in the chopped walnuts or pecans.

In a separate bowl, prepare the streusel by combining rolled oats, ¼ cup of flour, brown sugar, cinnamon, and melted butter. Mix until crumbly.

Divide the muffin batter evenly among the prepared muffin cups, filling each about ¾ full.

Sprinkle the streusel mixture generously over the muffin batter in each cup.

Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center comes out clean.

Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.