Heat the oil in a saucepan over high heat. Add the mushrooms. Cook, stirring occasionally, for 6 minutes or until mushrooms are tender. Stir in the vinegar and cook for 1 minute.
Combine 1 tablespoon of the stock with the cornflour and sugar in a small bowl. Stir until smooth. Stir cornflour mixture into the remaining stock then stir into the mushrooms. Add the bay leaf. Bring to the boil, stirring often.
Reduce the heat to medium. Cook, stirring occasionally, for 5 minutes or until sauce thickens slightly. Remove the bay leaf. Taste and season with salt and pepper.
Serve mushroom sauce with barbecued steak, chicken or sausages.