Make miso glaze by placing the first 7 ingredients in a small bowl and whisking to combine.
Heat the oil in a large skillet or wok set over medium-high. Add the prawns and stir-fry 2 minutes, or until just cooked through. Transfer prawns to a plate.
Add the miso mixture to the pan and bring to a simmer. Simmer 1 minute, and then return the prawns to pan. Heat through 1 to 2 minutes, and serve, sprinkled with green onion.
Note: To devein the prawns, use a small paring knife to make a lengthwise slit along the back of each peeled prawn. Pull out, or rinse out with cold water, the dark vein.