Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan.

In a large mixing bowl, combine the pumpkin puree, granulated sugar, brown sugar, vegetable oil, eggs, and vanilla extract. Blend until smooth and creamy.

In another bowl, whisk together the all-purpose flour, baking powder, baking soda, ground cinnamon, ground nutmeg, and salt.

Gradually add the dry mixture to the wet ingredients, stirring gently with a spatula until just combined.

In a separate bowl, combine the rolled oats, chopped nuts, and melted butter for the crumb topping until well incorporated.

Pour the batter into the prepared pan and spread it evenly. Sprinkle the crumb topping generously over the batter.

Place the pan in the preheated oven and bake for 30-35 minutes, checking for doneness with a toothpick.

Let the cake cool in the pan for about 10 minutes, then transfer it to a wire rack to cool completely.

Serve warm or at room temperature, optionally dusting with powdered sugar or adding whipped cream.