Start by draining all the oil from the canned sardines and set aside.
In a bowl, add the eggs and beat them well until fully dissolved.
Season them with one pinch of salt, one pinch of black pepper, one tomato chopped into small cubes, one onion chopped into small cubes and parsley to taste, mix all the ingredients together very well until incorporated.
Add the sardines and mix once again.
In a large skillet over medium-low heat, add the butter and stir until fully melted.
Pour in the mixture we made, spread it evenly on the skillet, cover the skillet and let it fry for 5 minutes.
After this indicated time, flip the omelet using a plate.
Fry on the other side for another 3 minutes.
Serve.