In a large pot, heat olive oil over medium heat. Add chopped onion and cook until softened, about 5 minutes.

Add turmeric, cumin, and mustard seeds to the pot and cook for 1 minute more, stirring constantly.

Add lentils and water to the pot. Bring to a boil, then reduce heat and simmer for 20-25 minutes, or until lentils are tender.

Add salt and remove from heat.

Puree the soup with a hand blender or in a regular blender until smooth.

Return the soup to the pot and add spinach. Cook over medium heat until spinach is heated and wilted.

Add lemon juice to taste. Add cilantro and heat briefly to warm and wilt it.

Serve hot over rice.