Place the pineapple juice, honey, ketchup, soy sauce, apple cider vinegar and mustard powder in a small saucepan with the heat off (this stops the ingredients from splattering).
Turn the heat to medium and stir to combine.
Bring to a gentle simmer, then add the cornflour and water mixture. Cook for 2–3 minutes, stirring often, until the sauce thickens slightly.
Remove from the heat and allow to cool – the sauce will thicken further as it stands.
Serve warm or cold.
