Salmon and dill burger with lemon and caper mayonnaise

Name: Salmon and dill burger with lemon and caper mayonnaise
Ingredients: 100g/3½oz smoked salmon; 500g/1lb 2oz salmon fillet, boned, skin removed and cut roughly into 4cm/1½in pieces; 125g/4½oz panko breadcrumbs; ½ lemon, juice only; 4 tbsp mayonnaise; small bunch dill, chopped; 2½ tbsp sunflower oil; salt and freshly ground black pepper; 1 tbsp capers, chopped; ½ small lemon, juice only; ¼ fennel bulb, very thinly sliced; 4 Romaine lettuce leaves, cut in half; 4 brioche buns, split in half and toasted on each side; 2 pickled dill cucumbers, thinly sliced