Preheat the oven to 425°F
Bake the potatoes for an hour until they are crispy on the outside and fluffy on the inside. Let cool. Remove the insides with a spoon and mash them, using a potato ricer or a fork, until they are very smooth.
Place the potatoes in a bowl and add the nutmeg, salt, a pinch of pepper, the egg yolk, and the ricotta. Mix these together to combine. Then place the mixture in the bowl of a stand mixer and, using the paddle attachment, add the flour on low-medium speed. Once the mixture has come together, remove it from the bowl and cut into fist-size balls. Let rest for 2 hours in the refrigerator, uncovered.
Spread the semolina flour in an even layer on a clean smooth surface.
Roll a ball in the palms of your hands and then form it into a rope about the width of a cigar. Cut each of these ropes into 1-inch pieces, the size of your knuckle (nocca in Italian). To cook, drop the gnocchi into salted boiling water. When the gnocchi float, they are done.