First make the paste: deseed the chilli and finely chop. Peel and grate the ginger & garlic. Discard he outer skin of the lemongrass, then finely chop the rest. Roughly ground the coriander seeds. Put all ingredients of the paste in a small food processor and whiz until you get a coarse paste.
Thinly slice the mushrooms and dice the tofu.
Heat a wok. When hot, add the paste and stir-fry for 30 seconds, then add the coconut milk & stock and bring to the boil.
When soup is simmering, add the mushroom and tofu, then add the tomato purée. Season to taste and simmer for 5 minutes.
Meanwhile, cook the noodles in another pan, drain and set aside. Slice the spring onions diagonally.