Preheat oven 375F. Line a standard 12-cup muffin tin with liners or cooking spray.
In a medium bowl, whisk together flour, baking soda, ground cinnamon, ground ginger, ground allspice, ground cloves, ground nutmeg, and salt until well combined.
In a separate large bowl, whisk together pumpkin puree, brown sugar, coconut oil, and vanilla extract until well combined. Mix in eggs, one at a time, then whisk until smooth. Add flour mixture to wet ingredients (pumpkin puree mixture), stirring using a spatula until just combined. Gently mix in the rolled oats and walnuts.
Using an ice cream scoop or big spoon, evenly distribute the batter among the 12 muffin cups (batter should fill 2/3rds to 3/4ths of the cup). Sprinkle tops of muffins with extra oats and chopped walnuts.
Bake muffins at 375F for 20-22 minutes or until they are springy to touch (or a tooth inserted in the center comes out clean). Allow muffins to cool about 5 minutes in the muffin tin before removing.