Heat the olive oil over medium-high heat in a medium stockpot. 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.
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. Remove the thyme sprigs and the bay leaf. Season to taste with salt and pepper.
Serve in bowls, with a dollop of Greek yogurt and a sprinkling of scallions.