For the salsa verde, place all the salsa verde ingredients into a blender and blend to combine. Add a little more oil and vinegar to thin the sauce to a dipping consistency if needed.
For the haddock tempura, place the vegetable oil into a deep, heavy-based saucepan and heat until a small cube of bread sizzles and turns golden when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Mix the flour and water together in a bowl, then fold in the whisked egg white.
Cut the haddock into strips, then dip into the batter and deep fry for 4-5 minutes, or until golden-brown and cooked through. Remove from the oil with a slotted spoon and drain on kitchen paper.
To serve, place the haddock tempura onto a serving plate with some lime wedges and the salsa verde sauce in a bowl, for dipping.