Step 1

Heat oil in a medium saucepan over medium. 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.

Step 2

To serve, divide soup among bowls. Drizzle with reserved coconut milk and top with more cilantro. Serve with lime wedges.

Step 3

Do Ahead: Soup (without toppings) can be made 3 days ahead. Let cool; cover and chill.