Heat the oven to 160°C (fan) / gas mark 4. Line a 23cm cake tin with baking paper.
Grind the oats in a blender for a few seconds to create a coarse flour, then mix with the spelled flour and baking powder.
Whisk the eggs and caster sugar together until thick and light. Add the milk, vanilla and melted butter gradually. Then fold in the flour, the finely chopped stem ginger and the orange peel.
Cut off the top and bottom of the rhubarb and cut into 2–3 cm pieces, then fold half into the cake mixture. Pour the mixture into the tin and place the rest of the rhubarb on top, spreading some oats on top.
Bake for 45 minutes until a knife/skewer comes out clean, and leave to cool in the tin for 20 minutes before removing.
Pour about 2 tablespoons of the ginger syrup over and serve warm with crème fraîche or cream.
