Start by making the paste for the soup. Peel and grate the garlic and ginger and slide them into a food processor. Trim the chilli and remove the seeds and pith if you prefer your food a little less spicy. Add half of the coriander and half of the basil, the chilli and the zest of the lime. Peel the shallots and add them to the processor, along with a good pinch of salt and pepper. Blend till everything is finely chopped.
Pour 1 tbsp olive oil into a large pan and bring to a medium-low heat. Scrape in the paste and fry for 5 mins, stirring frequently.
Pour 1 ltr of boiling water into the pan and crumble in the stock cube. Stir well and return to the boil. Reduce to a simmer and cook for 10 mins.
While the broth simmers, separate the pak choi leaves. Finely shred the stalks and roughly chop the leaves. Peel the carrot and use the vegetable peeler to shave both the carrot and courgette into ribbons, finely chopping the cores when you get down to them. Halve the heirloom tomatoes.
When the broth has cooked for 10 mins, add the pak choi, noodles and tomatoes. Cook for 3 mins, then stir through the carrot and courgette. Squeeze in the lime juice.
Ladle the Tom Yum soup into a couple of deep bowls. Serve scattered with the remaining herbs.