Place the cornflour into a bowl with half the five spice. Dust the beef cheeks well in the mixture.
Heat the oil in a large, heavy-based saucepan over medium-high heat. In batches, cook the beef cheeks for 5 minutes, turning until browned, then remove and set aside.
Pour the stock, remaining five spice and kecap manis into the pan and stir to combine. Return the beef cheeks to the pan and top with up enough water to completely cover the cheeks. Bring to a simmer, then reduce heat to low and cover with a lid. Cook for 2 hours, then add the mushrooms and any water if necessary. Return the lid. Continue to cook for a further 1 hour or until the cheeks are meltingly tender.
Serve with a serving suggestion as below.