In a saucepan, fry the chopped onion in vegetable oil for about 5 minutes, then add curry powder and cumin. Stir and fry for another minute.
Add the tomato and lentils, just cover with water and bring to a boil. Cover and simmer until lentils are cooked, about half an hour. Add water if necessary. Add the spinach leaves and cook until the spinach softens, 5 minutes. Season with chilli paste, if desired.