Melt the butter in a pot on medium high heat. Add the onion and cook, stirring occasionally until translucent. Sprinkle the onion with 1/2 tsp salt. This will help it cook evenly and release its juices. Add the garlic and cook for another five minutes, until everything is soft and a light brown. Add a little water to the pot if the onions are sticking to the bottom too much.

Add the ginger and curry powder and stir. (Tip: Keep your ginger in the freezer, it makes it really easy to grate and add to dishes like this.) Add some more salt to taste. Add the the pureed tomato and cook for about five minutes until the tomato is cooked and begins to separate from the butter (you can see this around the edges of the pan usually).

Add the lentils and water and bring the stew to a boil. Turn the heat down to low and let it simmer with a lid slightly askew for 20 to 30 minutes until the lentils are cooked through. Add the spinach and stir until it is wilted and incorporated. Cook for about 5 more minutes if using frozen. If using fresh once the spinach is wilted it's ready.

Taste the stew and add salt and fresh cilantro or any other seasonings according to your taste.