For the tempura batter: Mix together the all-purpose flour cornstarch rice flour, baking powder and salt in a bowl. Mix in the squid ink and add soda water to the desired consistency.
For the tartar sauce: Mix together the mayonnaise, cornichons, chives, dill, tarragon and lemon juice in a bowl.
For the fish and chips: Heat the peanut oil in a fryer or large heavy-bottomed saucepan to 325 degrees F.
Fry the black radish slices, turning them with a slotted spoon, until crisp, about 4 minutes. Remove to a paper towel and season with salt.
Bring the peanut oil up to 350 degrees F.
In batches, dip the cod into the tempura batter. Add to the hot oil and fry, flipping occasionally with a slotted spoon, until the fish is cooked through and the batter is crisp, about 6 minutes. Remove, drain on a towel and season with salt.
Serve with the tartar sauce and black radish chips on a large platter. Garnish the platter with chopped chives and fresh dill.