In a large pot heat the oil. Saute the onion over medium-high heat until soft, about 5 minutes.
Add the garlic, ginger, the chilli, cumin, garam masala, and coriander, curry powder. Cook until very fragrant, about 2 minutes.
Carefully pour in the tomatoes and scraping up any brown bits that formed on the bottom of the pan if any. Cook, stirring occasionally until the liquid has reduced and the tomato chunks have softened about 6 minutes.
Add the coconut milk and 100ml of water. Bring to a boil over high heat. Stir in the lentils and reduce the heat to medium-low. Cook until the lentils are tender, 20-25 minutes, stirring occasionally to prevent the lentils from sticking to the bottom of the pan. Add the salt.
Fold in the spinach and cook until wilted, 3 minutes, Adjust the seasoning if needed, more salt or spices.
Serve this easy curry alongside rice and with or naan.