Egg Mixture
Start by cutting the peeled potatoes into small dices.
Transfer all the potatoes to a saucepan, add enough water to cover all of them, add the salt and let them cook for 15 minutes.
After this indicated time, remove from the heat and drain all the water.
Into a skillet, add the unsalted butter or margarine and stir until fully melted. Add the onion chopped into dices and stir until golden brown, set aside.
Into a bowl, add the eggs, the milk, the salt and the black pepper, mix until combined.
Once you’ve done that, to a nice baking dish, transfer half of the cooked potatoes, the fried onion dices, the drained sardines, spread it out evenly.
Now add the other half of the potatoes, the egg mixture and the grated mozzarella cheese.
Take it to a preheated oven at 200°C (392°F) and let it bake for 40 minutes. Baking time may vary depending on the power of your oven.
Serve.