Heat oil in a pot over medium heat. Stir in onion and cook until soft, about 7 minutes, stirring occasionally. Add the lentils and water and bring to a boil, then reduce heat to low and simmer, uncovered, until lentils are tender, 20 to 25 minutes. Cooking times will vary depending on the freshness of the lentils.
Ladle a half cup of the soup liquid into a bowl and whisk in the flour to form a paste. Mix the paste into the soup. Add the spinach, lemon juice, cumin and salt. If you prefer a thinner soup, add a bit more water. Cook until spinach is wilted, about 5 minutes. Adjust salt and lemon to suit your taste.