Bring the broth to a boil and stir the tom yum paste through it, letting it cook out for two minutes until the oil lifts.
Add the chicken and poach it at a bare simmer for eight minutes, until the cubes are opaque right through the middle.
Tip in the baby corn, mushrooms, red onion and the chilies, and give them five minutes to soften without losing their bite.
Slide the tomato wedges in last and cook them for two minutes only, so they hold their shape instead of collapsing.
Off the heat, stir in the fish sauce and the lime juice, taste it for sour and salt, and scatter the cilantro over.
