Cover the pan with a lid and cook the onions until soft and caramelised.
Put the onions aside and use the same pan with a drizzle of olive oil to cook the burgers for 6-7 minutes on each side, season with salt and pepper to taste.
On each halved bun, put a few leaves of lettuce, a slice of tomato, a burger (still warm), caramelised onions, shavings of Grana Padano and serve immediately.