In a large bowl, combine the miso paste with 1 tbsp rice wine vinegar and 1 tsp sugar. Put the cod fillets in the marinade and turn over so they are evenly coated. Cover with cling film and chill for 1 hr.
Put the radishes in a small bowl with the black sesame seeds, the remaining vinegar and sugar, and a pinch of salt. Leave to pickle for at least 30 mins.
For the wasabi tartare sauce, combine all the ingredients and season well. Set aside or chill in the fridge until needed.
Meanwhile, put the sweet potato chips in a large saucepan filled with cold salted water. Bring to the boil and cook for 7-8 mins until slightly tender. Drain and leave to steam-dry for 2 mins.
Fill a large saucepan with the oil until two-thirds full. Place over a medium-high heat until the oil reaches 180C on a temperature probe (or until a piece of bread crisps up in 20 secs). Meanwhile, make the tempura batter by combining the flours and gradually whisking in the soda water. Whisk until the batter is lump-free, then chill until needed.
Once the oil has come to temperature, fry the sweet potato chips in batches until golden and cooked through, about 5 mins. Transfer to a baking sheet, season with salt and place in a low oven to keep warm while you fry the fish.
Tip some seasoned flour onto a plate and dip the cod fillets in it, patting off any excess. Dip into the batter and fry in the same oil as the chips for 3-4 mins, until the batter is golden and the fish is cooked. Drain on kitchen paper.
Serve the cod with the chips, wasabi tartare, pickled radishes and lime for squeezing over.