Cook the lentils in plenty of unsalted boiling water for about 12 minutes or until they are soft but holding their shape.
Meanwhile, melt the butter in a saucepan and gently cook the onion for about 8 minutes until it is softened but not coloured. Add the garlic and cook for 1 minute, then stir in the vinegar and all the spices except the garam masala and fry gently for 2 minutes.
Drain the lentils and stir them into the spice mix with the chopped spinach. Heat until the spinach has wilted and the lentils are hot. Season to taste, stir in the garam masala and serve immediately with plenty of chapattis and mango chutney.