In a large, heavy-bottomed pot, heat the olive oil over medium heat. Add the onions to the pot and stir. They should be sizzling but just barely. Continue to stir and sauté the onions until the are completely soft and starting to break down a bit, about 8 minutes.
To the onions, add the minced thyme, smoked paprika, and dried tarragon. Stir the spices in and cook until fragrant, about 1 minute.
Add the carrots and celery to the pot and stir. Season the whole thing with salt and pepper. Sauté the carrots and celery until barely softened, about 2 minutes. Add the garlic and stir until fragrant, about 30 seconds.
Add the lentils to the pot and stir to coat them with oil, spices and vegetables. Add the crushed tomatoes and diced tomatoes to the pot and stir to combine. Add the vegetable stock and stir one more.
Cover the soup and bring to a boil. Reduce the heat to a simmer and adjust the pot lid, slightly allowing some steam to escape. It should be sitting on top of the pot but slightly askew.
Simmer until the lentils are tender, stirring occasionally, about 25 minutes. Check the soup for seasoning, adjust accordingly and serve hot.