1. Heat the olive oil over medium-high heat in a medium stockpot.
2. Add the onions, carrots and celery, and season with salt and pepper. Cook, stirring often, until the vegetables have softened, about 5 minutes. Add the garlic, thyme and bay leaf, and continue cooking until the garlic is fragrant, 1 minute more.
3. Add the canned tomatoes with their juice, the chicken stock and the lentils. Bring to a boil, then reduce to a simmer. Cover and cook until the lentils are tender, about 30 minutes.
4. Remove the thyme sprigs and the bay leaf. Season to taste with salt and pepper.
5. Serve in bowls, with a dollop of Greek yogurt and a sprinkling of spring onions.