Method:
In a large saucepot, heat peanut oil over medium heat. Add onion, chicken or tofu, ginger and lemongrass; cook, stirring frequently, about 5 minutes or until onion is translucent. Stir in broth, fish sauce, kaffir lime leaves, sugar, crushed red pepper flakes and serrano peppers, if using. Bring to a boil. Reduce heat to medium-low, cover and simmer 10 minutes. Add tomatoes and mushrooms. Return to a simmer and cook 3 minutes longer or until heated through. Remove from heat and stir in lime juice and cilantro. If desired, remove and discard ginger, lemongrass and lime leaves before serving.