Heat the olive oil in a large pot over medium heat. Add the onion and cook for 5 minutes until it softens.
Stir in the garlic, parsnips, carrots, and potatoes. Cook for 3 minutes, stirring occasionally.
Add the lentils, vegetable broth, thyme, salt, and black pepper. Bring the mixture to a boil.
Reduce the heat to low, cover the pot, and simmer for 30 minutes until the lentils and vegetables are tender.
Remove the lid and simmer uncovered for 5 more minutes to thicken the stew slightly. Stir in the parsley just before serving.