Melt the butter in a large frying pan over a low heat, add the pancetta and cook, stirring occasionally, for 10–12 minutes until crisp.
Stir in the cream and sage, increase the heat to high and bring to the boil. Boil until slightly thickened. Stir in the fontina and Parmesan, then remove from the heat. Stir until most of the cheese has melted, then season with salt and pepper.
Cook the gnocchi in a large saucepan of salted boiling water until they rise to the surface: according to the packet instructions for bought gnocchi or for 3–4 minutes if using homemade. Drain thoroughly and stir into the sauce. Serve immediately.