Wash the red lentils in several changes of water, and rinse. Combine the lentils, onion, potatoes, garlic, and water in a large soup pot.
Cover, and bring to a boil. Reduce the heat and simmer until everything is tender, 15-20 minutes. Remove from the heat.
In a small skillet on low heat, warm the oil until it is hot but not smoking. Add the cumin, turmeric, and salt. Cook, stirring constantly, for 2-3 minutes. Take care not to scorch the spices. Add this to the soup.
Puree the soup with an immersion stick blender until smooth. Add the lemon juice. Reheat gently and season with salt and pepper, and more lemon juice to taste.