Wash the pumpkin thoroughly with the skin on, cut into pieces, and steam.
Cool the steamed pumpkin, remove the skin, and mash.
Mix oatmeal, pumpkin, egg, milk, and erythritol to make the batter.
Pour the batter into a cake mold.
Make a crumble topping with Greek yogurt or cream cheese and oatmeal as desired.
Place the crumble topping over the batter.
Bake in a preheated oven at 180°C for 25-30 minutes.
Let it cool, remove from the mold, and serve.