To make the laksa paste, whizz all the ingredients and 4 tbsp of water in a blender until smooth.
To make the curry, put the sweet potato, swede and carrot cubes in a saucepan. Cover them with water and cook until tender. Drain, reserving 600ml (1pt) of the cooking liquid.
Heat 2 tbsp of oil in a wok, add the tofu and fry until crisp. Remove and lay on absorbent paper. Add a further 2 tbsp of oil to the wok and heat, spoon in the laksa paste and fry for 2 minutes to release the aromas. Add the coconut milk, stock, soy sauce, sugar and reserved cooking liquid.
Bring to the boil, then simmer for 7 minutes. Add the green beans and cook for a further 3 minutes, until the beans are tender.
Meanwhile, put the noodles in a large heat-proof bowl, cover with boiling water and set aside for 10 minutes.
Drain and divide among 4 deep warmed bowls. Top with the tomatoes, vegetables and tofu. Ladle over the soup, garnish with the herbs and spring onions and serve with a wedge of lime.