Preheat oven to 180°C.
Beat eggs and milk in a large bowl and add cheese.
Heat half the oil in an ovenproof frying pan over low heat and cook onion and garlic, stirring until soft (approximately 5-6 minutes). Remove and add to the egg mixture with the potato.
Wipe pan clean, then return to low heat with remaining oil. Pour in frittata mixture and using a palette knife, gently pull away from sides of the pan, allowing uncooked egg to run to edges.
Once cooked around the outside (about 2-3 minutes), transfer pan to the oven and bake for 10-15 minutes until set.
Turn out and cool. Serve with salad or steamed vegetables.