Heat the oven to 350° F degrees. Pierce the potatoes with a fork and bake until soft, or for about 90 minutes. While still hot, cut the potatoes in half and scoop out the middle. When cooled, pass the potato through a potato grater. Add the white pepper and then the egg yolk. Add about 1 cup of flour to the mixture. Mix thoroughly with a fork or pastry cutter and then form a ball. On a well-floured work surface, knead and fold mixture until the flour is incorporated, adding another 1 to 2 cups along the way. The dough should not feel sticky, so add a little more flour if necessary. Cover with flour and let rest for five minutes. Divide dough into 8 pieces. Roll out each piece into a long tube of about 1 inch thick and then cut into lengths about 1 inch long. Next, use a fork to make each piece into the shape of gnocchi, by rolling each piece up and down the tines of the fork. (If you do not plan to use the gnocchi within an hour or so, freeze them until ready.)
For the Sauce:
In a large skillet, over medium heat, sauté the onion in the olive oil until golden. Add the tomato, salt and pepper to taste. Reduce the heat to low and simmer for about 30 minutes, stirring regularly, until reduced.
Bring a large pot of salted water to a boil. Cook the gnocchi for about 2 minutes and drain. Place in a baking dish and mix in most of the sauce. Cover with the sliced mozzarella and the remaining sauce. Bake at 350 degrees for about 15 minutes, or until the mozzarella is melted and golden.