Mix the ricotta with the grated parmesan and mozzarella. Take 2 thick slices white bread and spread the cheese mixture over one of them. Top with the Gorgonzola, broken into pieces, then close the sandwich with another slice of bread.
Spread the outsides of the sandwich, top and bottom, with the mayonnaise. Fry the sandwich in the butter for 3-4 mins each side, pressing the sandwich down gently with your spatula as it cooks to make sure it melts evenly inside and stays together.