Heat the oil in a medium dutch oven over medium heat. Add the onion and bell pepper and sauté, stirring occasionally, until golden brown, about 10 minutes.
Add garlic, ginger, curry powder, and red pepper flakes and cook until fragrant, about 2 minutes. Add the lentils and cook for about 1 minute more, stirring occasionally.
Add tomatoes, cilantro, 2½ cups water and season with salt and pepper. Set aside a few tablespoons of the coconut milk and add remaining coconut milk to dutch oven.
Bring the mixture to a boil and reduce heat to simmer, stirring occasionally, until lentils are soft but not mushy, 20-25 minutes. Taste and adjust salt and pepper as needed.
To serve, divide soup among bowls. Drizzle with reserved coconut milk and top with plenty of cilantro. Serve with lots of lime wedges.