This is hands down one of the absolute favorites of the CC's in the Thai repertoire. It was requested so frequently by the CC's friends once that it was practically a standby as appetizer before "other" meals.
It also happens to be extraordinarily easy provided you have the right ingredients.
At its heart, it involves the simmering of extraordinarily fresh ingredients and then topping with some protein and aromatics.
Once again, it's a meta-recipe where you can have tom yum goong (shrimp) or tom yum gai (chicken) or tom yum talay (mixed seafood.)
David Thompson's recipe is ultra-minimalist and ultra-purist and it would work if the CC lived in a tropical climate and had access to the very best of produce in large quantities. It also eliminates shallots which is fine for royal cuisine but that's not what you're getting on the street.
Not having access to that quality of produce perenially, the CC's version is closer to that of Kasma Loha-Unchit's which tweaks the parameters to get the same taste profile.
The soup is an exercise in freshness. Simmer it at the last moment. Ideally, the boiling hot soup should be poured over the fresh meat/seafood and served at once. They will cook in the boiling broth and be both perfectly cooked and tender at the same time.
The CC once had a the sister of a friend who refused to eat barely cooked shrimp until we persuaded her to try it. Then she wanted seconds and thirds (of which alas, there were none!) A convert to the excellence of barely cooked seafood.
The existence of this soup on menus across the country may be a cliché but there's a solid reason it is one. It's a masterpiece of flavor.
3 cups water
1 small block tamarind
3-5 kaffir lime leaves
3 pieces lemongrass (sliced at a diagonal, pounded lightly)
1" galangal (sliced, pounded lightly)
3-4 Thai chillies (or to taste)
1 tbsp nahm prik pao (roasted chili paste/jam)
2 shallots (sliced lengthwise into thin slivers)
1 small tomato (sliced into quarters)
1/2 cup straw mushrooms
fish sauce (to taste)
Note 1: It's best to use water rather than broth. You get a very sharp clear taste profile. Broth muddies the waters so to speak. Think of it as the difference between limpid watercolors and an oil painting.
Note 2: The fresher the chicken/shrimp/seafood and also the aromatics, the more this soup will shine.
Note 3: The chicken should be sliced thinly against the bias. The shrimp can be left whole. The seafood should also be sliced finely.
Note 4: Since the galangal, lemongrass and kaffir lime leaves are not eaten, technically you could strain it. That's not the Thai way and yet the CC has seen it done in a superb "fancy" Thai restaurant in Manila.
Note 5: Scallions are fairly traditional too but the CC doesn't like the clash between the shallots, scallions and cilantro. He likes the sharper watercolor flavors.
Note 6: There are excellent brands of nahm prik pao on the market. No artificial ingredients, etc. You could be a purist and make it but the CC finds that these are not just acceptable but positively excellent.
Note 7: The CC would like to draw your attention to a powerful concept. The idea of a background "mellow" sourness (tamarind) allied with a perfumed "fresh" sourness (lime). You will see this idea as a conceptual framework all along Southeast Asia whether it's kudampuli + lime (South India) or dried raw mango (aamchur) + lime (North India) or tamarind/vinegar/raw mango + calamansi (Philippines.) You'll see it Italy too (vinegar + lemon). It's an extraordinarily potent idea.
Smash the galangal and lemongrass lightly. Add them, the chillies and kaffir lime leaves to the water.
In a separate bowl, pour hot water over tamarind and let it soak for 20 minutes. Strain through a fine sieve into the same bowl of liquid. Discard the remnants.
Bring the liquid to a boil. When it is boiling, turn the heat down to a low simmer. Add the nahm prik pao and let it simmer for 10 minutes. Add fish sauce and check to taste. It should be aromatic, spicy and salty (and a little tart and sweet from the tamarind.)
Add the shallots, tomatoes, and mushrooms. Bring to a boil again for 2 minutes.
Turn off the heat. Squeeze the lime juice into it.
Pour over the chicken/seafood in individual bowls. Top with the cilantro leaves.
Serve at once with jasmine rice.