Heat the olive oil in a large pot over medium heat.
Add the chopped onion and sauté until it becomes translucent, about 5 minutes.
Add the diced carrots and celery, and sauté for an additional 5 minutes.
Add the minced garlic, ground cumin, and ground turmeric to the pot. Sauté for 1 to 2 minutes until the spices release their aromas.
Add the rinsed and drained lentils to the pot, along with the bouquet garni.
Pour in the vegetable broth and bring to a boil.
Reduce the heat and simmer, covered, for about 25-30 minutes, or until the lentils are tender.
Remove the bouquet garni from the soup.
Add the fresh baby spinach and cook for 2 to 3 minutes, until just wilted.
Season the soup with salt and pepper to taste.
Add the lemon juice if desired, for a touch of freshness and acidity.
Serve the soup hot, garnished with chopped fresh herbs like parsley, cilantro, or chives.