Barramundi burgers with tartare sauce

Name: Barramundi burgers with tartare sauce
Cuisine: American
Ingredients: 4 x 100g skinless barramundi fillets, pinboned; 1/3 cup (50g) plain flour; 1 egg, lightly beaten; 2/3 cup (50g) dry breadcrumbs; 1/3 cup (80ml) olive oil; 4 bread rolls, split; Watercress sprigs, sliced tomato and sliced avocado, to serve; 1 cup (300g) whole-egg mayonnaise; 2 tablespoons lemon juice, plus lemon wedges to serve; 2 tablespoons chopped cornichons (see note); 1 tablespoon chopped capers; 1 tablespoon chopped flat-leaf parsley; 1/2 teaspoon Dijon mustard