Heat the olive oil over medium heat in a heavy soup pot or Dutch oven.

Add the onion, carrot, celery, and garlic and cook stirring for about 5 minutes. Add the bay leaf, thyme sprigs, and a generous pinch of salt, and continue to cook for another 10 minutes until tender. Add the rinsed lentils and stir so the lentils take on the onion mixture flavor. Add the tomatoes stirring for a couple of minutes so everything incorporates. Add the veggie stock and salt to taste. Bring to a boil.

Reduce the heat to a simmer, partially cover, and cook for about 45 to 50 minutes until the lentils are soft. Add the mustard and stir, fold in the spinach and cook for 5 minutes. Taste and add more salt and pepper if needed. Remove the bay leaf and thyme sprigs and serve.