Heat the tom yum paste in a medium saucepan. Add the hot chicken stock, kaffir lime leaves, lemongrass, lime juice, fish sauce, chilli and sugar. Bring to the boil and then simmer over a medium heat for 5 minutes to infuse all the flavours and make a spicy stock.
Add the chicken, halved button mushrooms and tomato wedges and simmer for another 2 minutes to warm through. Divide the soup between 2 bowls, then garnish with some fresh coriander leaves to serve.
For more ideas on what to do with leftover chicken, see Chicken recipes.
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Please note, tom yum paste is only available in selected stores.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.