Heat the groundnut oil to 180°C in a large, heavy-based saucepan.
Prepare the tempura batter by whisking the soda water into 75g of the flour in a steady, slow stream, making sure that the batter is smooth. The consistency should be that of single cream. Season the batter with salt and pepper.
Make sure the vegetables are dry, then toss them in the remaining 50g of flour. Shake off any excess and then dip into the batter and deep-fry in batches. Deep-fry the vegetables for 2-3 minutes before draining on kitchen paper.
Meanwhile, whisk together the ingredients for the sauce apart from the wasabi until the sugar is dissolved. Place the vegetable tempura on kitchen paper placed on the plates. Serve the sauce in little side pots and place a little wasabi in each. Serve immediately.
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