Sauté Aromatics: In a large pot, heat vegetable oil over medium heat. Add onion, garlic, and ginger. Sauté for 2–3 minutes until fragrant.
Add Lemongrass and Chilies: Stir in lemongrass and sliced Thai chilies; cook for another 1–2 minutes to release flavors.
Add Stock and Coconut Milk: Pour in chicken or seafood stock and coconut milk. Bring to a gentle simmer.
Season the Broth: Stir in fish sauce, lime juice, and sugar. Taste and adjust seasoning with salt and pepper.
Add Vegetables: Add mushrooms and red bell pepper. Simmer for 5 minutes until vegetables are tender.
Cook Shrimp: Add shrimp and cook for 3–5 minutes until pink and fully cooked.
Add Fresh Herbs: Remove from heat and stir in chopped cilantro and basil. Remove lemongrass pieces before serving.
Serve: Ladle soup into bowls, garnish with extra herbs and sliced chilies if desired.