Rinse the red lentils under cold water until water runs clear, then drain and set aside.
Heat olive oil in a large pot over medium heat. Add chopped onion and sauté for about 5 minutes until softened and translucent.
Add the minced garlic, ground cumin, turmeric, and cinnamon to the pot. Cook for 1-2 minutes, stirring frequently, until fragrant.
Pour in the rinsed lentils and vegetable broth. Stir to combine.
Bring the mixture to a boil, then reduce heat to low, cover, and simmer for about 25 minutes, or until lentils are soft and soup is slightly thickened.
Remove from heat and use an immersion blender to puree the soup until smooth. Alternatively, carefully transfer batches to a blender and blend until smooth, then return to the pot.
Stir in lemon juice. Season with salt and black pepper to taste.
Heat through for 2-3 minutes and serve hot, garnished with chopped parsley or cilantro if desired.