Heat the oil in a large stock pot over medium heat. Add the onion, celery, and carrots and cook, stirring frequently, until the vegetables are soft and the onions are translucent, about 3-4 minutes. Add the minced garlic and cook for another minute, until fragrant.

Pour the vegetable stock over the vegetables, then add the red lentils, bay leaves, turmeric, and cumin and stir well. Turn the heat up to medium-high to bring the soup to a boil, then reduce the heat to medium-low to simmer. Cover and simmer for 12-15 minutes, until the lentils are soft.

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

Transfer 2 cups of soup to a blender and blend on high until smooth, then pour it back into the soup and stir to combine. Alternatively, use an immersion blender to partially blend the soup to your desired consistency.

Add the spinach, if using, and cook to wilt for 2-3 minutes. Taste and add more salt and pepper as needed. Serve with fresh cracked black pepper, cilantro, or parsley.