Heat the vegetable oil in a wok or very large saute pan over very high heat until very hot. Add the ginger, garlic, and shallots, and saute for 1 minute. Add the prawns and cook just until they start to turn pink, 2 to 3 minutes.
Add the mirin and lemongrass, and cook, stirring often, 2 to 3 minutes longer. Add the stock and chile sauce, and cook until the prawns are just cooked through. Season, to taste, with soy sauce. Serve warm.