To make the batter, pour the flour in a bowl. Mix the egg in a separate bowl and add the water. Slowly pour the water/egg mixture into the flour and lightly mix. The batter should still be slightly lumpy. Season.
To make the broth, pour all the ingredients into a pan, season to taste and bring to a simmer.
Cook the noodles in boiling salted water for 3-4 minutes then drain and refresh in cold water.
Pour the vegetable and sesame oil in a pan and heat up. Test the temperature by adding a small drop of tempura batter in the oil, if it rises to the surface then the oil is ready.
Individually coat the vegetables in the batter and deep fry until crisp. Leave the prawns until last.
To serve, place some soba in the bottom of a bowl, top with the tempura, pour the broth into the bowl and finally finish with some spring onions.