Preheat the grill to high. Peel the cucumber into ribbons with a vegetable peeler. Thinly slice the tomato.
Step 2 — Prep fish
FISH OPTION Whisk half the ayam goreng paste* and a generous drizzle of oil in a large bowl until combined. Add the fish to the ayam goreng oil and turn to coat.
Step 3 — Cook fish
Heat a medium frypan over medium heat. Add the fish, including the ayam goreng oil in the bowl, to the pan (make sure you add all the ayam goreng oil for maximum flavour). Cook for 3½-4 mins each side until browned and cooked through. Remove from the pan.
Step 4 — Grill buns
Meanwhile, halve the buns through the middle. Put on an oven tray, cut-side up, and grill for 2-3 mins until lightly toasted.
Step 5 — Make salad
Put the wasabi (if you don't tolerate heat, omit the wasabi) in a large bowl, add 2 tsp white vinegar and 2 tsp extra virgin olive oil, season with salt and whisk to combine. Add the salad leaves and cucumber and toss to coat.
Step 6 — Assemble burgers
Divide the fish, tomato and three-quarters of the salad among the bun bases and drizzle with any pan juices, then sandwich with the bun tops. Serve the burgers with the remaining salad.