In a saute pan add the EV olive oil and the onion. Let the onion saute until it become translucent. Drain the oil from the onion and let it cool.
In a bowl, add he ground beef, the onion and salt and pepper as desired. Mix all the ingredients well and form 4 hamburgers.
Grill the hamburgers to your desired cooking level.
In the last two minutes add a slice om mozzarella on each burger and let it melt just a little.
Put each burger in a bun and garnish with tomatoes and arugula.