Heat the oil in a large pan and cook the mustard seeds and mace until the seeds pop. Lower the heat and add the chillies, garlic, ginger, garam masala and onion. Fry gently for a moment or two, taking care not to burn the spices. Add the sweet potatoes and parsnips and stir well. Bring to boiling point, then cover and simmer for a few moments. Add the tomatoes and lemon juice. Stir again thoroughly, then simmer for 15 minutes. Taste and season. If it needs pepper, add some, but it should be spicy and hot enough.