Cook the potatoes in a large pot of salted, boiling water for about 20 minutes, or until tender. Drain and mash in a large bowl. Let cool.

Cook the spinach in a large saucepan over low heat for 5 minutes, or until tender. Drain, squeezing out the excess water.

Process the spinach in a food processor or blender until pureed. Mix the spinach puree into the potatoes. Stir in the flour, egg yolk, and salt to make a smooth dough.

Flour your hands and shape the dough into "sausages" about 2/3 inch (1.5 cm) in diameter. Cut into 3/4-inch (2-cm) lengths. Dust with flour to prevent them from sticking.

Melt the Taleggio or Fontina with the milk in a small saucepan over low heat, stirring constantly, for 5 minutes, to make a smooth cream.

Melt the butter with the sage in a small saucepan over medium heat for about 2 minutes, or until the butter begins to brown.

Cook the gnocchi in small batches in a large pot of salted boiling water with the oil for about 3 minutes, or until they bob to the surface.

Remove with a slotted spoon and transfer to a serving dish with the cheese sauce. Drizzle with the sage butter and season with salt and pepper. Serve hot.