Heat the olive oil in a large pot over medium heat. Add the onion and cook until it softens. Stir in the garlic and cook for one minute more.
Add the carrots, parsnips, and sweet potato. Cook for about five minutes, stirring occasionally. Sprinkle in the thyme, cumin, and paprika, then stir to coat the vegetables.
Pour in the diced tomatoes with their juices and the vegetable broth. Add the lentils. Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 30 minutes, stirring now and then, until the lentils and root vegetables are tender.
Taste and season with salt and black pepper before serving.