Bring water to a boil and add about 1 tablespoon salt. Lightly crush the lemongrass, garlic, shallots, and slice the young galangal, then add them to the pot. Wait for the water to boil.

Once boiling, add the shrimp heads, followed by sliced onion and crushed cilantro roots (if using). Do not stir. Let simmer for about 5 minutes.

Add the prepared mushrooms and sliced tomatoes. Season with about 2 tablespoons sugar and 3 tablespoons fish sauce. Let cook for 8–10 minutes.

Add chilies and one-third of the green onions and cilantro. Squeeze in the juice of half a lime. Let simmer for about 5 minutes.

Pound dried chilies with Thai chili paste. Add the shrimp, then add the pounded chili mixture (do not stir, just gently press the shrimp down so they're submerged). Add another third of the green onions and cilantro. Season with 1 tablespoon sugar, 2 tablespoons fish sauce, and about 1/2 teaspoon seasoning powder (optional). Let cook for 5–10 minutes.

Add the remaining green onions and cilantro. Squeeze in the juice of the remaining half lime. Add about 2–3 tablespoons larb seasoning powder (recommended: any roasted rice larb seasoning). Let simmer for 3 minutes, then turn off the heat. Taste and adjust with more lime juice or seasonings as desired. Serve hot in bowls.