Chop onion and crush garlic.
Heat oil in a large pot over medium-high heat.
Sauté onions and garlic for 5 minutes, or until fragrant and translucent.
Add ginger, celery, and carrots to the pot and sauté for an additional 5 minutes.
Pour in red lentils, vegetable broth, coconut milk, and red curry paste. Add fish sauce if desired.
Bring to a boil, then reduce heat and simmer for 20 minutes.
Allow soup to cool before transferring to a blender or using an immersion blender to puree.
Transfer soup back to pot and heat through.
Taste and adjust seasoning as needed.
Serve with green onions, cilantro, and lime wedges.