Heat the olive oil in a large saucepan over medium heat. Add the onion and garlic and cook until softened, about 5 minutes. Stir in the cumin, coriander, curry powder and turmeric and cook for 1 minute.
Stir in the vegetable broth and lentils. Bring to a boil, reduce heat to low, cover and simmer until the lentils are tender, about 20 minutes.
Stir in the diced tomatoes with juice. Simmer for an additional 10 minutes.