Use the back of a knife to bruise the lemongrass and chop it in 2-3" pieces.
Use the back of a knife to bruise the kaffir lime leaves. (If using dried leaves, skip this step).
Saute the lemongrass, ginger, kaffir lime leaves and nam prik pao over medium heat for 2-3 minutes.
Stir in the water, coconut sugar, fish sauce, MSG (optional) and lime juice.
Bring to a boil, reduce the heat and simmer, uncovered, for 10 minutes.
At this point, you can strain the kaffir lime leaves and lemongrass out as they can be tough.
Add the mushrooms and tomatoes.
Add additional salt (or MSG) to taste, if desired.
Remove from heat, add shrimp, cover and let sit for 5-10 minutes, until the shrimp has turned pink.
Serve over noodles with chopped cilantro and fresh squeezed lime.
Notes
For a spicier soup, just add more chili paste.
If the soup is too sour or tangy for you, add a little more sugar. And, if it’s too sweet or salty, you can add more lime juice.
I used frozen shrimp but defrosted them before adding them to the soup. You can do this by leaving them in the fridge overnight or running cold water over them in a colander.
You can also use fresh shrimp. If they're shelled, just peel and devein them first.
You can also use prawns in place of shrimp in this recipe.
Be careful not to overcook the shrimp in the soup as they will get tough.
The lemongrass and kaffir lime leaves are added to the broth for flavor. Since they tend to be tough, I recommend straining the broth to remove them.
For a creamy texture, stir in some coconut milk or evaporated milk at the end.
If you have leftovers, store the broth and noodles separately. If combined, the noodles will soak up the broth and get mushy.
You can leave out the noodles and serve this soup with sticky rice instead.