Trim tips and root ends of lemongrass stalks and remove and discard tough outer layer. Using a meat mallet or the side of a knife, smash lemongrass to flatten it; tie stalks into a knot and set aside. Pour stock into a 2-quart saucepan and bring to a boil. Add lemongrass and half the lime leaves, reduce heat to medium-low and simmer until fragrant, about 5 minutes.

Remove and discard the lemongrass and lime leaves. Increase heat to high and stir in mushrooms add chile paste, to taste. Boil for about 1 minute; add shrimp and fish sauce and cook until shrimp are cooked through, about 45 seconds to a minute.

To serve. Combine remaining lime leaves with chiles, scallions ad lime juice in a serving bowl. Ladle soup into serving bowl, stir and serve immediately.
Add rice as option. No rice for no carb alternative.