While the lentils are cooking, melt 85g butter in a deep frying pan. Add the onions, season generously with salt and fry for 15≈minutes, until soft but not coloured. Finely slice the chard stalks, then add to the pan with the garlic and remaining teaspoon of whole cumin seeds, and fry for another seven to eight minutes, until the stalks have softened but still have some bite. Stir through the nigella seeds and keep warm.