Step 1
Preheat the oven to 350 degrees F (175 degrees C).
Step 2
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 nice smooth consistency. Place potatoes in a bowl and refrigerate until they are cool 15 to 20 minutes; this will help keep them fluffy.
Step 3
Dust 2 sheet pans with flour and set aside.
Step 4
Mix cooled potatoes and 1 1/2 cups flour together on a clean work surface. Knead until dough is soft and smooth 4 to 5 minutes adding only enough flour so dough comes together and is not tacky.
Step 5
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.
Step 6
Preheat the oven to 350 degrees F (175 degrees C).
Step 7
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.
Step 8
Melt butter for cheese sauce in a medium saucepan over medium heat. Whisk in flour until golden brown about 3 minutes. Add chicken stock and continue to whisk until it thickens about 3 minutes. Add Fontina cheese and Parmesan cheese; stir using a spoon until melted. Stir in sage and nutmeg. Stir in cooked gnocchi and transfer to a casserole or baking dish.
Step 9
Bake in the preheated oven until top is golden brown and bubbly 15 to 20 minutes.