In a large lidded Dutch oven or stockpot, heat the olive over a medium flame. Add the onion, carrot, and celery and saute until soft, about 8 minutes. Carefully pour in the tomatoes. Simmer for 5 minutes, until reduced, pressing down on the tomato chunks with the back of your spatula to crush them.
Stir in the bay leaf, thyme, soy or Worcestershire, lentils, and stock. Bring to a boil, then reduce the heat to medium-low and simmer, partially covered, until the lentils are al dente, about 30 minutes.
Remove cover and skim off any residue from the top of the broth. Stir in the Dijon mustard and salt until well incorporated - it will make the broth slightly creamier and more opaque. Taste for seasoning and add more salt as necessary.
Gently fold in the spinach leaves and cook until soft and wilted, about 2 minutes.
To serve, ladle the soup into bowls and drizzle with a little olive oil. Crusty bread is never a bad thing either.