Heat the olive oil in a large pot over medium heat. Add the chopped onion and cook until it softens, about 5 minutes.
Stir in the minced garlic and tomato paste. Cook for 1 minute until the paste darkens slightly.
Add the diced carrots, potatoes, and parsnip. Stir to coat the vegetables with the oil and tomato mixture.
Pour in the vegetable broth, then add the rinsed lentils, dried thyme, and bay leaf. Season with a pinch of salt and pepper.
Bring the pot to a boil, then lower the heat and cover. Simmer for 30 to 35 minutes, stirring occasionally, until the lentils and root vegetables are tender.
Remove the bay leaf. Taste and add more salt or pepper if needed before serving.