3 large stalks fresh lemongrass, ends trimmed and smashed with a meat mallet or heavy knife
4 cups chicken stock
1 small white onion, chopped
12 fresh Kaffir lime leaves, or juice from 3 fresh limes
1 cup canned straw mushrooms, drained
2-4 tablespoons roasted Thai chili paste
8 ounces medium shrimp, peeled and deveined
1 tablespoon vegetable oil
1 1/2 tablespoons fish sauce
4-6 Thai chiles, stemmed and with a meat mallet or heavy knife
3 scallions, cut into 1" pieces
Heat a stock pot to medium-high; add oil and onions. Cook onions for 5 minutes, or until they become fragrant and translucent. Add stock and bring to a boil; add lemongrass stalks and 6 lime leaves (or the lime juice). Reduce heat to medium-low and simmer for 5 minutes.
Remove and discard lemongrass and lime leaves and increase heat to high. Stir in mushrooms and chili paste; boil for 1 minute. Add shrimp and fish sauce and cook until shrimp are just cooked through, about 45 seconds. Combine remaining lime leaves with chiles, scallions, and lime juice in a serving bowl. Pour soup into serving bowl, stir, and serve with steamed rice, if you like.