Heat the oil in a large stock pot over medium heat. Add the onion, celery, and carrots, cooking while stirring frequently until soft and the onions are translucent, about 3-4 minutes.

Add the minced garlic and cook for another minute until fragrant.

Pour in the vegetable stock, then add the red lentils, bay leaves, turmeric, and cumin. Stir well.

Turn the heat up to medium-high to bring the soup to a boil, then reduce heat to medium-low and let it simmer. Cover and simmer for 12-15 minutes, until the lentils are soft.

Remove the lid, then stir in the lemon juice, lemon zest, salt, and pepper.

Transfer 2 cups of soup to a blender and blend on high until smooth. Pour it back into the pot and stir to combine.

If using, add the spinach and cook until wilted, about 2-3 minutes. Taste and adjust seasoning if needed. Serve hot!