Prep the shrimp (optional): Peel and devein the shrimp. But for extra flavor, I recommend keeping the heads and shells on. Set aside.
Boil the herbs: Bring the water to a boil in a pot or wok over medium heat. Add the shallots, lemongrass, galangal, and kaffir lime leaves. Simmer to release their aroma.
Add seasonings: Stir in the chili paste, palm sugar, fish sauce, salt, and lime juice. Let it return to a gentle boil.
Make it creamy: Add evaporated milk and mushrooms. Simmer for a few minutes until the broth turns creamy and slightly pink.
Cook the shrimp: Add the tomatoes and shrimp. Simmer just until the shrimp turn pink and are cooked through, about 1–2 minutes.
Garnish and serve: Ladle into bowls and top with culantro, coriander, and green onions. Serve warm.