Place the two tomatoes, garlic cloves, shallots, lime zest and Thai Tom Yum spice in a food processor and blend to a smooth paste.
Heat the peanut oil in a large saucepan and add the spice paste. Fry until fragrant – about 4 minutes. Add the stock and bring to a boil over high heat. Reduce the heat and simmer for about 15-20 minutes to infuse the broth with flavour. Strain into a clean saucepan, pressing the mixture to release as much flavour as possible. Return to the heat.
Place noodles in a separate large bowl and cover with boiling water. Let stand until tender (about 10 minutes) and then divide among your serving bowls.
Meanwhile, add the prawns, tomatoes, fish sauce, palm sugar and lime juice to the broth and simmer for about 3 minutes, until the prawns are just cooked through and the tomatoes softened. Taste and adjust the seasoning accordingly, adding more lime for sourness, fish sauce for saltiness or palm sugar for sweetness.
Divide the soup among your serving bowls, and top with sliced chilli, spring onions and coriander leaves to serve.