Pour the broth into a soup pot. Add the lemongrass, galangal or ginger, and makrut lime leaves, if using. Bring to a gentle boil over medium-high heat, then reduce the heat and simmer for 8 minutes to infuse the broth.
Add the mushrooms and tomato. Simmer for 4 to 5 minutes, until the mushrooms are tender and the tomato begins to soften.
Stir in the Thai roasted chili paste, fish sauce, and sugar until the chili paste dissolves. Taste the broth carefully and adjust with a little more fish sauce for saltiness or sugar to soften the tart flavor, if needed.
Add the shrimp and simmer gently for 2 to 3 minutes, just until they turn pink, opaque, and curled. Do not boil hard or cook them longer than needed.
Remove the pot from the heat. Stir in the lime juice and sliced chiles, if using. Remove and discard the lemongrass, galangal or ginger slices, and lime leaves before serving if desired.
Ladle the soup into bowls and top with cilantro. Serve immediately, with cooked jasmine rice on the side if you would like a more filling dinner.