Remove and discard the white stalks from the silverbeet. Steam leaves until just limp then drain and chop finely.
Melt the butter in a saucepan and add the mustard seeds, fenugreek seeds, cumin, nutmeg, chilli powder and sugar. Stir over medium heat about 2 minutes until the mustard seeds start to pop.
Stir in the yoghurt and spinach. Simmer until heated through, about 1 minute.