Heat the olive oil in a large pot over medium heat. Add the onion and cook for 3 minutes until it softens.
Stir in the garlic, carrots, parsnips, and potato. Cook for 5 minutes, stirring occasionally.
Add the lentils and barley to the pot. Pour in the vegetable broth and add the thyme sprigs, salt, and pepper.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 35 minutes until the barley and lentils are tender.
Remove the thyme sprigs before serving.