Heat a saucepan, then fry the curry paste for 1 min. Add the coconut milk and half a can of water, then bring to the boil. Simmer for 5 mins until slightly thickened. Throw in the mushrooms and simmer for another 2 mins. Stir in the drained noodles, beansprouts and most of the chilli and spring onions, then heat for another min. Serve in deep bowls, scattered with the rest of the chilli and onions.