Heat the olive oil or coconut oil in a large pot over medium heat.

Add the chopped onion and sauté for 3-4 minutes until softened and translucent.

Stir in the minced garlic and grated ginger, cooking for another 1-2 minutes until fragrant.

Add the ground turmeric, cumin, coriander, cinnamon, and crushed red pepper flakes (if using), stirring to coat the onions and release the spices' aroma.

Add the rinsed red lentils, diced carrot, and diced celery to the pot, stirring to combine.

Pour in the vegetable broth and water, and bring the mixture to a gentle boil.

Reduce the heat to low, cover the pot, and let the soup simmer for 20-25 minutes, or until the lentils are soft and fully cooked.

Using an immersion blender, blend the soup partially to create a creamy texture while keeping some chunks intact. Alternatively, transfer half the soup to a blender, blend until smooth, and return it to the pot.

Stir in the lemon juice, lemon zest, and coconut milk (if using), adjusting the seasoning with salt and freshly ground black pepper to taste.

Simmer for an additional 5 minutes to allow the flavors to meld together.

Ladle the soup into bowls and garnish with fresh parsley or cilantro before serving.