Heat the olive oil in a large pot over medium heat. Add the onion, carrots, and potatoes. Cook for 5 minutes, stirring now and then, until the onion turns soft.
Stir in the garlic and cook for 1 minute until it smells fragrant.
Add the lentils, vegetable broth, salt, and pepper. Bring the mixture to a boil, then lower the heat to a simmer. Cover the pot and cook for 20 minutes.
Stir in the chopped cabbage and thyme. Simmer uncovered for another 15 minutes, stirring occasionally, until the lentils and potatoes are tender and the cabbage is cooked through.
Taste and add more salt if needed. Serve hot.