- Preheat the oven to 180°C / 150°C fan-forced.
- Heat ½ the oil in a large ovenproof frypan over a medium heat. Add the onions and stir gently for 5 minutes or until golden. Add the sugar and continue to cook for 3-4 minutes or until caramelised. Remove from the heat and cool.
- Beat the eggs and milk in a large bowl to combine. Stir in ½ of the Mozzarella, garlic, and season with salt and pepper. Add the potato, the onion, and gently toss to combine.
- Heat the remaining oil in the same ovenproof frypan (which should be wiped clean) over a low heat. Pour the mixture into the pan, and then arrange the potato in an even layer. Cook for 2-3 minutes, and then sprinkle over the remaining cheese.
- Transfer into the oven and cook for 15 minutes or until set, puffed and golden. Remove from the oven, carefully transfer onto a plate, and serve the frittata warm.