Place the oil in large skillet and set over medium heat. Add the onion and ginger and cook 3 to 4 minutes. Mix in the flour and curry powder and cook 1 minute more. Slowly stir in the coconut milk and then add the stock and lime juice. Bring to a simmer and simmer 5 minutes. Mix in the prawns, snap peas and cilantro, if using. Simmer a few minutes more, or until the prawns are heated through and the snap peas are just tender. Season with salt and serve.
Note: Peeled, cooked prawns are available in the seafood department.