Trim tip and root ends of lemongrass stalks and remove tough outer layer.
Using a mallet smash the lemongrass to flatten it; tie stalks into a knot; set aside.
Pour broth into 2 qt saucepan and bring to boil.
Add lemongrass, lime leaves, ginger, and chili halves reduce heat and simmer 7-10 minutes until fragrant.
Strain broth, discarding lemongrass, lime leaves, ginger and chili halves.
Return to pan and increase heat to high.
Stir in mushrooms and chili paste and boil for 1 minute; add shrimp, fish sauce, and sugar and cook until shrimp are just cooked through, about 1-2 minutes.
Place chili pepper slices, scallions, and lime juice into serving bowls and top with shrimp, broth, and cilantro leaves.
Adjust flavors by adding more fish sauce or lime juice just 1/4 teaspoon at a time