Curry: Skim the coconut cream off the top of the coconut milk and heat or alternatively heat a few tablespoons of vegetable oil in a heavy saucepan over medium heat.
Add the red curry paste and fry until fragrant. Add the coconut milk and water.
Add the tuna, ginger, lime leaves and lemon grass. Bring to a gentle boil stirring well to break up the tuna chunks. Simmer for a half hour or longer until the herbs have released their aroma and the oil comes to the surface.
Season with fish sauce and sugar to taste. If the tuna was very salty the sauce might not need any additional seasoning.
Garnish: Boil the noodles according to instructions until soft. Drain and put into a bowl of cold water.
Pull a few fingers full of noodles out at a time and wrap them around your fingers creating little nests. Squeeze the little nest gently to rid it of all the water. Lay on a serving platter. Repeat with the rest of the noodles.
Serve noodles. garnishes and the heated curry sauce so everybody can assemble their own plates according to their taste.