Heat oil in a medium saucepan over medium heat. Cook onion, stirring often, until softened and golden brown, 8-10 minutes.
Add garlic, ginger, curry powder, and red pepper flakes and cook, stirring until fragrant, about 2 minutes. Add lentils and cook, stirring, 1 minute.
Add tomatoes, ½ cup cilantro, a generous pinch of salt, and 2½ cups water; season with pepper.
**Set aside ¼ cup coconut milk for serving and add remaining coconut milk to saucepan.
Bring mixture to a boil; reduce heat and simmer gently, stirring occasionally, until lentils are soft but not mushy, 20-25 minutes. Season soup with more salt and pepper if needed.
To serve, divide soup among bowls. Drizzle with reserved coconut milk and top with more cilantro. Serve with lime wedges.