Simmer lemongrass, garlic, and ginger in chicken broth for 30 minutes. Strain and discard solids.
Heat oil in a large soup pot over medium heat. Add chicken and cook for 5 minutes.
Add mushrooms and sauté for another 5 minutes.
Stir in fish sauce, red curry paste, and lime juice.
Add broth and coconut milk. Simmer on low for 15–20 minutes.
Add sliced red onion; cook 5 more minutes.
Remove from heat and stir in half the cilantro.
Serve with lime wedges, jalapeños, and remaining cilantro.
