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.