Prepare the chicken – Toss the chicken pieces with cornflour until lightly coated. This helps seal in moisture and gives the sauce something to cling to later.

Mix the sauce – In a small bowl, whisk together ketchup, vinegar, soy sauce, sugar, cornflour, and water until smooth. This prevents lumps and ensures even thickening.

Heat the pan – Warm the oil in a large pan over medium-high heat. The pan should be hot enough to sizzle without smoking.

Cook the chicken – Add the chicken in a single layer and cook, turning, until lightly golden and just cooked through. Remove to a plate to avoid overcooking.

Soften the vegetables – Add the onion and pepper to the same pan. Stir-fry until softened but still slightly crisp, keeping colour and texture.

Combine and sauce – Return the chicken to the pan, add the pineapple, then pour in the sauce. Stir gently as it bubbles and thickens, coating everything evenly.

Finish and serve – Cook for a final minute until glossy and heated through. Taste and adjust with a splash more vinegar or sugar if needed.