Preheat the oven to 180℃ / Fan 160℃ / 355℉ / Gas 4. Prepare the cake tin. Grease and fully line the cake tin with grease proof paper.
Prepare the rhubarb & arrange in the cake tin. Remove the ends of the rhubarb & discard. Wash and dry the rhubarb. Cut the rhubarb into pieces and arrange in the bottom of the prepared cake tin. We arranged ours as a wheel with 6 longer pieces stretching from the edge to the centre of the tin, and smaller pieces of rhubarb laid adjacent to the longer lengths to fill in the triangles. Some pieces may need to be cut on an angle to ensure the whole surface is covered as much as possible. Cut the left over rhubarb into pieces about 3cm long and place into a small pan ready to make the compote (optional). Set the pan aside.
Scatter the finely chopped stem ginger evenly over the arranged rhubarb.
Make the sponge batter. Place the soft butter and sugars into a good sized bowl and beat together with a wooden spoon or electric beaters until very pale and fluffy. Add the salt, stem ginger syrup and finely grated root ginger. Beat again to combine. Gradually add the beaten eggs a little at a time, beating well after each addition, (see note b below). Sieve the flour and ground ginger into the mixture. Use a spatula or large metal spoon to fold this in gently. Mix in a little milk until you have a nice dropping consistency.
Fill the cake tin. Spoon the prepared batter over the arranged rhubarb and stem ginger. Spread the batter out gently to level, aiming not to disturb the fruit. Use the back of a spoon to make a slight indentation to the centre of the batter. This will help it bake level.
Bake. Place the cake tin in the centre of the oven and bake for about 50 minutes until a cake skewer inserted in the centre comes out clean. You may need to rotate the cake tin after 40 minutes of baking. Once baked, remove from the oven and place onto a cooling rack.
Turn out the cake. Allow the cake to cool for 5 minutes before removing from the tin. Invert a cooling rack on top of the cake tin. In one movement, swiftly turn the cake tin and cooling rack upside down. Remove the cake tin and greaseproof paper. The arranged rhubarb will now be the top of the cake. Allow the cake to finish cooling.