1. Rinse the lentils under cold water until the water runs clear.
2. In a large pot, bring the water to a boil. Add the lentils and reduce the heat to a simmer. Cook for about 20 minutes, until the lentils are tender.
3. In a separate pan, heat the vegetable oil over medium heat. Add the chopped onion and cook until translucent, about 5 minutes.
4. Add the minced garlic, cumin, turmeric, and coriander to the onions. Cook for another 2 minutes, stirring frequently.
5. Add the cooked onion mixture to the pot of lentils. Stir in the salt and chopped spinach. Cook for another 5 minutes, until the spinach is wilted.