1. Make a base stock for the soup by boiling 2 litres of water with the carrots, celery, onion, coriander seeds and stalks from the fresh coriander, dried red chillies, lime leaves, lemongrass and shrimp paste (if using) and palm sugar.
2. Bring to a boil whilst you cook the noodles in a separate pan of boiling water. Boil according to the pack instructions and drain once soft.
3. In the empty pan toss in the tomatoes, mushrooms, veggies, prawns and most of the red chilli and spring onion.
4. Once the stock has been boiling for 10 minutes strain it into the veggie pan. Turn the heat on and bring to a simmer to cook the prawns.
5. Stir in the noodles and taste. Adjust the seasoning with lime juice and fish sauce. It should be hot, sour, spicy and salty.
6. Serve sprinkled with the fresh coriander leaves and remaining red chilli and spring onions.