Sprinkle the sliced par-boiled potatoes and raw onion with Potato Spice. Heat the oil in a non-stick frying pan and add the butter. Add potatoes and onions at the same time. Cover and cook slowly over medium heat (it is really stewing rather than frying), for about 10 minutes, stirring occasionally, until the vegetables are tender but not brown. Add the sliced baby marrows for the final 5 minutes of cooking. Drain the vegetables in a colander over a bowl, reserving the oil for buttering the muffin pans. Allow to cool. In another bowl beat the eggs, then mix the vegetables with the eggs and leave to soak for about 15 minutes. Adjust the oven rack to middle position and preheat oven to 180°C. Spoon the potato and egg mixture into the muffin pans. Bake for 25 - 30 minutes. Leave to cool before lifting out. Garnish with crumbled feta cheese, snipped chives and freshly ground black pepper.