Slice the "Murazzano" cheese into thin slices. Then prepare the omelette base mixture.
Beat the eggs with a whip. Add the "Murazzano" cheese, the flour, salt and pepper. Stir the ingredients delicately.
Cook the onion cut into thin slices with the butter and two tbsp. of extra virgin olive oil until it is lightly coloured. Add the spinach cut julienne, cover and let cook over very low heat for approx. 10 minutes.
Pour the mixture of eggs and "Murazzano" cheese into a rectangular oven baking pan, lined with oven paper. Carefully choose the size of the baking-pan: the layer of the mixture for the omelette must be approx. 5 mm thick. Preheat the oven to 170°C and insert the baking-pan for 10-15 minutes.
Once cooked, take the omelette out of the oven, cover it while still hot with the spinach and roll it up. Then wrap the roll in tin foil and put it in the refrigerator.
Once cooled, unwrap the roll and cut it into slices about 1 cm thick and place on the plate.