For the mushrooms, mix together the flour and Chinese five spice powder in a bowl until well combined. Season with a pinch of salt, or to taste.
Dredge the oyster mushrooms in the spice mixture until completely coated.
Heat the oil in a deep heavy-based frying pan until a breadcrumb sizzles and turns brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.) Carefully lower the coated mushrooms into the oil and fry for 4-5 minutes, or until crisp. Remove from the pan using a slotted spoon and set aside to drain on kitchen paper. Keep warm.
For the dressing, heat the chilli, star anise, ginger, rice vinegar and sugar in a pan until simmering. Continue to cook for a further 4-5 minutes, or until the mixture resembles syrup.
Remove the pan from the heat and set aside until cooled, then whisk in the oil until well incorporated into the mixture. Stir in the chopped plums until coated in the mixture.
Stir the watercress and cucumber into the dressing mixture until coated in the dressing.
To serve, divide the dressed salad equally among four serving plates. Sprinkle over the pickled ginger. Spoon over the crisp mushrooms. Drizzle over any remaining dressing.