Omelette with onions

Thinly slice the onions and blanch them lightly. Put half the butter in a pan with the onions. Cook over very low heat but do not brown the onions. As soon as the onions barely start to be golden, add the eggs previously beaten with salt and pepper. Stir for a second with a wooden spoon and, once the omelette starts to clot, shake lightly the pan in order to detach it from the bottom. Then, turn the omelette tossing it or turning it upside down onto a large plate. Add the remaining oil, or butter, in the pan and brown the other side.