Preheat your oven to 350°F (175°C). In a mixing bowl, combine flour, sugar, and melted butter until the mixture resembles coarse crumbs. Press this dough evenly into the bottom of an 8x8 inch baking dish. Bake for 10 minutes until lightly golden.

In a large bowl, whisk together pumpkin puree, sweetened condensed milk, egg, cinnamon, nutmeg, and ginger until smooth and well combined. Pour the pumpkin filling over the pre-baked crust.

In a separate bowl, mix together brown sugar, flour, and cold butter using a pastry cutter until crumbly. Fold in oats if desired.

Sprinkle the crumble mixture evenly over the pumpkin filling.

Bake in the preheated oven for 45-50 minutes until the filling is set and the topping is golden brown.

Remove from oven and let cool for 15-20 minutes before serving.