In a large bowl, mix bananas, cucumbers, mint, cilantro, ginger, and red chile pepper to make salsa.
In a small bowl, blend lime juice, fish sauce, and brown sugar until sugar has dissolved. Thoroughly mix into the salsa.
Bring a large saucepan of lightly salted water to a boil. Place prawns in the water, and cook 3 minutes, or until flesh is opaque. Serve with the banana salsa.