For the fritters, combine the cornflour, flour and egg in a bowl and add enough sparkling water to make a light batter the consistency of double cream.
Heat the oil in a deep heavy-based saucepan until a breadcrumb sizzles and turns brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.) Dip the sprigs of parsley and coriander in the batter and deep fry in the oil for 2-3 minutes, or until golden-brown and crisp. Remove with a slotted spoon and drain on kitchen paper.
For the crisp trout, heat the olive oil and butter in a frying pan and season the fish with salt and freshly ground black pepper. Fry the fish for 2-3 minutes on both sides, or until golden-brown and cooked through.
For the dipping sauce, combine all of the dipping sauce ingredients in a small serving bowl.
Place the trout on a serving plate and arrange the herb fritters alongside. Serve with the dipping sauce to the side.