Place the white fish in a food processor with a little salt and whiz to a coarse pulp. Transfer to a bowl and incorporate the crabmeat, mayonnaise, parsley, mustard and lemon zest; mix well. Add the breadcrumbs, mix again and season to taste.
Divide the mix into 4 evenly sized burgers.
Heat a thin film of oil in a nonstick frying pan, add the burgers