Heat the oil in a nonreactive saucepan. Add the onion and carrot and cook over moderately high heat, stirring, until the onion is softened. Stir in the garlic and cumin and cook for 2 minutes. Add the lentils, chicken stock and thyme and bring to a simmer. Cover and simmer over low heat, stirring, until tender, about 40 minutes. Stir in the vinegar and season with salt and pepper. Reserve 1/4 cup of the cooking liquid. Cover the lentils and keep warm.