The base of a good Thai Tom yam soup is a paste called Nam prik pao or Thai Chilli paste made from roasted garlic, chillies, shallots and as with many Thai base recipes dried shrimp. A commercially made paste is available and perfectly acceptable but I think for the most vibrant authentic taste it is best made fresh ingredients.
Nam prik pao
10 large Red Thai Chillies, de-seeded and membranes removed
10 cloves of Garlic, peeled
8 large Banana Shallots, peeled and roughly chopped
200 gr Galangal, peeled and sliced
100 gr Dried Shrimps, soaked overnight in a little water
50 gr Light Brown Sugar
8 Lemongrass, peeled and chopped
6 tablespoons Tamarind Paste
4 tablespoons Rice Wine Vinegar
3 tablespoons Vegetable Oil
400 ml Water
Heat the oven to 375 F / 190 C / Gas mark 5. Place the shallots, galangal, garlic and chillies on a tray and drizzle with the oil, place in the oven and roast for forty-five minutes until soft and caramelised. Remove from the oven, allow to cool and place in a food processor. Drain the shrimp and add along with the lemongrass to the roasted shallots and spices. Blitz to make a paste. Place the paste, sugar, water, tamarind paste and vinegar in a medium-sized heavy-bottomed pan over medium heat. Bring to a boil then reduce heat to the gentlest
simmer, stirring frequently to prevent sticking and burning, and reduce the mixture until it becomes smooth and thick. Remove from heat and allow to cool. Store in an airtight container or sterilised jar in the refrigerator for up to two weeks.
If you really like heat do not remove the seeds or any membranes from the chillies.