Preheat the oven to 160C. Grease a deep 20cm round cake tin and line the base with baking parchment.
Place the chopped rhubarb pieces into a bowl and add the sugar and orange zest, mix together and set aside while you prepare the crumbe and cake batter.
To prepare the crumble, in a bowl combine the flour, oats, sugar and ginger, then add the butter and rub together until the mixtures resembles breadcrumbs. Keep aside until required.
Next, prepare the cake batter. In a large bowl, beat the caster sugar, butter and vanilla extract until pale and fluffy. Add the eggs and pour the flour. Stir until well combined without flour pockets in the batter.
Add the crème fraiche to the mixture and again fold until smooth.
Pour the cake batter into the cake tin. Top with the rhubarb mixture and finish by adding ¾ of the crumble. (While baking the crumble will sink for most of it, so keep ¼ of the crumble and sprinkle on the cake 45 minutes while baking.)
Place the cake in the oven and Bake for about 1 hr 15 mins or a skewer inserted into the middle of the cake comes out clean.
Allow to cool in the tin for 10 mins before carefully transferring onto a wire rack to cool completely. Enjoy on the day or store in an airtight container for up to 3 days.