For the tempura vegetables, whisk the flour, water and salt together in a large bowl until it reaches the consistency of single cream, adding more water if necessary.
Pour the vegetable oil into a large saucepan and heat until a small cube of bread turns golden in 30 seconds. (CAUTION: Hot oil can be dangerous. Do not leave unattended.) Dip the vegetables in the batter and lower into the hot oil. Deep fry until golden-brown and crisp, about 4-5 minutes. Remove with a slotted spoon and drain on kitchen paper.
For the dipping sauce, place all the ingredients into a bowl and mix well.
To serve, pour the dipping sauce into a small bowl on a serving plate, surrounded by the tempura vegetables.