Soften the noodles in a large pan of boiling water to cover, pushing the noodles under the water. Set aside for 5-10 minutes, until the noodles are completely soft. Mix the curry powder, turmeric, sugar, sesame oil, soy sauce and 1 tbsp water in a bowl.
Heat a wok until very hot. Add the sunflower oil, onion and pepper. Stir-fry for 3-4 minutes until softened and starting to brown in places. Drain the noodles and add to the pan, along with the sauce mixture and bean sprouts. Stir-fry for a further 3-4 minutes, tossing everything through the sauce, until hot. Adjust the seasoning with a little more soy or sugar, if you like, and scatter over the chilli, if you like more spice.