In a large pot, heat the olive oil over medium heat. Add the diced onion, garlic, carrot, and celery. Sauté until the vegetables start to soften, approximately 5 minutes.
Stir in the ground turmeric and cumin, letting the spices bloom in the heat for about 1-2 minutes.
Add the rinsed lentils and pour in the vegetable broth. Bring the mixture to a boil, then lower the heat and simmer uncovered for about 30-35 minutes, or until lentils are tender.
Once the lentils are tender, stir in the chopped spinach and lemon juice. Cook for another 5 minutes until the spinach is wilted. Season with salt and pepper to taste.
Ladle the soup into bowls and garnish with fresh herbs.