Preheat the oven to 350 degrees F (175 degrees C).

Fill a large saucepan with water. Add potatoes and 1 teaspoon salt; bring to a boil. Continue to boil until potatoes are very soft, about 15 minutes. Drain and run through a food mill or a ricer for a smooth consistency. Place potatoes in a bowl and refrigerate until they are cool, about 15 to 20 minutes.

Dust 2 sheet pans with flour and set aside.

Mix cooled potatoes and 1 ½ cups flour together on a clean work surface. Knead until dough is soft and smooth, about 4 to 5 minutes, adding enough flour so the dough is not tacky.

Cut dough into 4 equal parts. Roll 1/4 of the dough into a 3/4-inch log. Cut the log into 3/4-inch pieces. Roll the pieces over a gnocchi board or the back of a fork to create ridges and place onto a floured sheet pan in a single layer. Repeat with the rest of the dough. Let rolled gnocchi dry for 1 hour before cooking.

Preheat the oven to 350 degrees F (175 degrees C).

Bring a large pot of lightly salted water to a boil. Cook gnocchi in the boiling water until they float to the top, 2 to 4 minutes. Drain and set aside.

Melt butter in a medium saucepan over medium heat. Whisk in 4 tablespoons of flour until golden brown, about 3 minutes. Add chicken stock and whisk until thickened, about 3 minutes. Add Fontina and Parmesan cheese; stir until melted. Stir in sage and nutmeg.

Transfer cooked gnocchi into the cheese sauce. Combine well and transfer to a casserole or baking dish.