Heat the olive oil in a large pot over medium heat. Add the onion and cook until softened, about 5 minutes. Stir in the garlic, ginger, curry powder, cumin, turmeric and cayenne pepper; cook for 1 minute.
Stir in the vegetable broth and lentils. Bring to a boil then reduce heat to low and simmer until lentils are tender, about 30 minutes.
Stir in the diced tomatoes with juice and cook for an additional 10 minutes. Serve warm.