For the tartare sauce, whisk the mustard, vinegar and egg yolk together in a bowl until well combined. Slowly pour in the oil in a thin stream, whisking the mixture continuously, until it forms a mayonnaise. If the mixture gets too thick or splits, add a little of the water and keep mixing.
Once all of the oil has been added, season with salt and freshly ground black pepper and add lemon juice, to taste. Stir in the rest of the tartare sauce ingredients. Chill until needed.
Meanwhile, for the fish, cut the fillets into thin slices. Sprinkle the gluten free flour onto a plate. Beat the eggs in a bowl. Sprinkle the gluten free breadcrumbs onto a separate plate. Dredge the fish goujons first in gluten free flour, dusting off any excess, then dip them in the egg, then rolls them in the gluten free breadcrumbs until coated.
Heat a little oil in a frying pan until a breadcrumb sizzles and turns golden-brown when dropped into it. Fry the fish goujons, in batches, for 2-3 minutes, or until golden-brown and cooked through. Remove from the pan with a slotted spoon and set aside to drain on kitchen paper. Keep warm. Repeat the process with the remaining goujons.
To serve divide the fish goujons between two plates. Garnish the plates with a lemon wedge and salad leaves. Place the tartare sauce into dipping bowls and serve on the side.