In a large pot, bring Water to a gentle boil.
Bruise and smash the bottom sections of the Lemongrass, slice the Galangal into thin rounds, and tear the Kaffir lime leaves; add all three to the simmering water along with thinly sliced Shallots and simmer for 8–10 minutes to infuse the broth.
Add the Thai bird's eye chilies whole or lightly crushed (depending on how spicy you like it) and simmer 1–2 minutes to mellow their heat into the stock.
Stir in the Thai roasted chili paste (nam prik pao), Fish sauce and a small pinch of Sugar; taste and adjust to balance salty, sweet and spicy notes.
Add halved Straw mushrooms and wedges of Medium tomato and cook until the mushrooms are heated through and the tomatoes begin to soften, about 2–3 minutes.
Add the Medium shrimp and cook just until they turn pink and opaque, about 2–3 minutes more; avoid overcooking.
Remove the pot from heat and stir in Fresh lime juice to brighten the soup—start with the juice of one lime and add more to taste.
Ladle into bowls and garnish with roughly chopped Cilantro and thinly sliced Spring onions (scallions). Serve immediately while hot; adjust chili, fish sauce or lime to taste at the table.
