Heat the vegetable oil in a deep-sided frying pan, add the garlic and cumin seeds and fry for 1 minute to release their fragrance. Add the spinach, raise the heat and turn the spinach in the oil with the garlic and cumin. Add the lemon juice and zest and cook until the spinach has wilted. Stir in the sugar, yoghurt and black pepper and adjust the seasoning. Serve hot.