Melt the butter in a pan then stir in the flour. Cook for 1min then gradually whisk in the milk. Slowly bring to the boil, whisking all the time, then simmer for 3-4 mins until the sauce is smooth and thickened.
Stir in the fontina, taleggio and gorgonzola cheeses and simmer the sauce, stirring all the time, until the cheeses have melted. Season lightly with salt and freshly ground black pepper. While the sauce is simmering, boil the gnocchi in a large pan of lightly salted water for 3-4 mins until they just rise to the surface, or according to the packet instructions.
Drain well and place in a shallow gratin dish. Pour over the sauce and scatter over the grated Parmesan cheese. Place under a medium-hot grill for 4-5 mins until golden and bubbling.