First, make the dill mayonnaise. Put all the ingredients into a bowl and combine. Cover and chill in the fridge until needed.
In another bowl, mix the breadcrumbs and lemon zest. Put the flour and eggs into two more bowls. Place the pieces of haddock into the bowl with the flour and toss to coat. Then one by one, dip the fish pieces first in the egg, then into the crumb mixture, and transfer to a baking tray.
Preheat the oven to 220ºC, fan 200ºC, gas 7. Heat the oil in a large, deep, heavy-based frying pan. The oil should be about 2cm deep. Meanwhile, cover a tray with kitchen towel. When the oil is hot, test with a cube of bread, it should sizzle but only begin to turn golden after a minute. It the bread browns instantly, the oil is too hot.
Carefully fry the fish in batches for 3-4 minutes until golden. Transfer to the kitchen towel. Once all the fish has been fried, transfer to a baking tray and bake for 3 minutes until cooked through. Serve with the dill mayonnaise and lemon wedges.