Start the sweet onion base: Heat 2 tablespoons of neutral oil in a large frying pan over medium heat. Add the sliced onions and a generous pinch of salt. Cook, stirring occasionally, for about 15 minutes. You'll see the onions turn from stark white to a soft, translucent gold, and the air will fill with a mellow, sweet fragrance. They should be limp and glossy, not crispy.

Build the teriyaki sauce: In a small bowl, whisk together the soy sauce, honey, rice vinegar, sesame oil, minced garlic, and grated ginger. Pour this mixture over the cooked onions. Turn the heat up to medium-high and let it bubble gently for 3–4 minutes. The sauce will darken and thicken slightly; you should smell the sharpness of the vinegar mellowing into the honey.

Thicken to a glaze: Reduce the heat to low. Give your cornflour slurry a quick stir, then drizzle it into the pan while stirring constantly. Within 30 seconds, you'll see the sauce turn from a watery sheen to a glossy, coat-the-back-of-a-spoon consistency. Remove from heat and set aside.

Prepare the chicken: Place the chicken breasts between two sheets of baking parchment and gently pound them to an even thickness of about 2 cm. This ensures they cook evenly. Season both sides with a pinch of salt and white pepper. The flesh should feel firm but not hard after pounding.

Sear the chicken: Wipe out your pan (or use a fresh one) and heat 1 tablespoon of neutral oil over medium-high heat until it shimmers. Lay the chicken in the pan—listen for a robust sizzle. Cook for 4–5 minutes per side. The underside should be deep golden brown and crusty, and the chicken should release easily from the pan. If it sticks, it needs another minute.

Rest and slice: Transfer the chicken to a cutting board and let it rest for 5 minutes. This is crucial—the juices need time to settle back into the meat. When you slice it, the knife should glide through without resistance, and the interior should be white and moist. Slice into thin strips against the grain.

Combine chicken with sauce: Return the sliced chicken to the pan with the sweet onion teriyaki sauce. Toss everything together over low heat for 1–2 minutes until every piece is coated. The chicken will look lacquered, and the sauce will cling to the meat rather than pooling at the bottom.

Toast the sub rolls: Split your rolls and place them cut-side up under a hot grill (broiler) for 1–2 minutes, until the edges are golden and the interior is warm and slightly crisp. Be careful—they go from toasted to burnt in seconds.

Assemble the subs: Lay a handful of shredded lettuce on the bottom half of each roll. Pile on the chicken and onion mixture generously. If using cheese, lay a slice on top of the hot chicken so it melts slightly. Close the sub and press down gently. The roll should feel warm, the sauce should threaten to drip, and the lettuce should add a cool crunch.