Heat the oil in a large non-stick saucepan over a high heat. Season, then brown the chicken thighs all over for 4-5 mins. Remove with tongs and set aside on a plate.

Lower the heat, add the onion to the pan and cook for 8-10 mins until softened, adding a splash of stock if needed to prevent burning. Stir in the garlic and cook for 1 min until fragrant, then add the yellow pepper.

Stir in the oregano, brown sugar, mustard, Worcestershire sauce, tomato purée, diet cola, remaining stock and ketchup. Bring to the boil, then reduce the heat, add the chicken back to the pan and simmer for 10 mins, covered, until the chicken is cooked through.

Turn the heat up and bubble for 10 mins until the sauce has reduced and thickened slightly. Serve sprinkled with the spring onion over rice or pasta.