In a large pot or Dutch oven, heat the vegetable oil over medium heat. Once the oil is hot, add the Thai red curry paste. Sauté for about a minute until it becomes fragrant.
Next, toss in the sliced onion, minced garlic, and galangal (or ginger). Sauté these aromatics for about 2-3 minutes until the onions are translucent and the garlic is fragrant.
Pour in the reduced-sodium chicken broth and bring it to a gentle simmer. Add the lemongrass, fish sauce, Asian chili sauce, brown sugar, dried basil, and torn kaffir lime leaves (or bay leaves and lime zest). Stir well to combine all the flavors.
Let the soup simmer for about 10-15 minutes. This allows all the wonderful flavors to meld together. If you have a lid, cover the pot partially to keep the heat in while allowing some steam to escape.
After the broth is infused, it’s time to add the shrimp, sliced mushrooms, and tomato wedges. Cook for about 3-5 minutes, or until the shrimp turn pink and the mushrooms are tender.
Once the shrimp and mushrooms are ready, stir in the lime juice. Taste and adjust the seasoning, adding more fish sauce or lime juice if desired.
Ladle the hot soup into bowls and garnish with fresh cilantro. Serve immediately, and enjoy the explosion of flavors in every spoonful!