Preheat your oven to 350°F (175°C). Grease the baking pan or line it with parchment paper for easy removal.
In a large bowl, combine the flour, rolled oats, brown sugar, granulated sugar, salt, cinnamon, and nutmeg. Mix well until combined.
Pour in the melted butter and stir until the mixture resembles coarse crumbs. Reserve about 1 cup of the crumble mixture for topping. Press the remaining mixture into the bottom of the prepared baking pan evenly.
In another bowl, whisk together the pumpkin puree, evaporated milk, eggs, vanilla extract, and baking powder until smooth.
Pour the pumpkin mixture over the crust in the baking pan, spreading it evenly.
Sprinkle the reserved crumble mixture over the pumpkin filling evenly.
Bake in the preheated oven for 30-40 minutes, or until the top is lightly golden and the pumpkin filling is set. A toothpick inserted in the center should come out clean.
Once baked, remove from the oven and allow to cool in the pan for about 15 minutes. Then, transfer to a wire rack to cool completely.
Cut into bars and serve. Enjoy!