Make the Gnocchi Dough:

1. Cook the Potatoes: Start by placing the peeled and cut potatoes in a large pot and covering them with cold water. Add a pinch of salt to the water and bring it to a boil. Cook the potatoes for about 15-20 minutes or until they are fork-tender.
2. Mash the Potatoes: Drain the potatoes and allow them to cool slightly. Then, use a potato masher or ricer to mash the potatoes until they are smooth. Let the mashed potatoes cool completely before proceeding with the dough.
3. Prepare the Gnocchi Dough: Once the potatoes are cool, transfer them to a clean surface or large bowl. Sprinkle the flour over the mashed potatoes and make a well in the center. Crack the egg into the well, and sprinkle in the salt, pepper, and nutmeg (if using).
4. Combine the Dough: Gently mix the ingredients together with your hands or a spoon until the dough starts to come together. Knead the dough for about 5-7 minutes, adding more flour if necessary, until it is smooth and elastic. Be careful not to overwork the dough, as this can make the gnocchi tough.

Shape the Gnocchi:

5. Divide and Roll the Dough: Divide the gnocchi dough into smaller portions. Roll each portion into a long, thin log, about ½ inch thick. Use a sharp knife to cut the logs into bite-sized pieces, approximately 1 inch long.
6. Form the Gnocchi: To give the gnocchi its traditional shape, gently press each piece with the back of a fork to create small ridges. This helps the gnocchi hold the sauce better and gives it an authentic look.
7. Cook the Gnocchi: Bring a large pot of salted water to a boil. Working in batches, carefully drop the gnocchi into the boiling water. When the gnocchi rise to the surface (about 2-3 minutes), use a slotted spoon to remove them and transfer them to a plate. Set aside while you prepare the sauce.

Make the Tomato Sauce:

8. Cook the Onion and Garlic: In a large skillet, heat the olive oil over medium heat. Add the chopped onion and cook for 5-7 minutes, or until softened and translucent. Add the minced garlic and cook for another 1-2 minutes until fragrant.
9. Add the Tomatoes: Add the crushed tomatoes (or chopped fresh tomatoes) to the skillet. Stir in the dried oregano and basil, and season with salt and pepper to taste. If the sauce is too acidic, add a pinch of sugar to balance the flavors.
10. Simmer the Sauce: Reduce the heat and let the sauce simmer for 15-20 minutes, stirring occasionally. The sauce should thicken slightly and develop a rich flavor. Adjust seasoning as needed.

Combine the Gnocchi with the Tomato Sauce:

11. Toss the Gnocchi in the Sauce: Gently add the cooked gnocchi to the tomato sauce and toss to coat them evenly. Allow the gnocchi to simmer in the sauce for a few minutes to absorb the flavors.
12. Add the Mozzarella: Tear or slice the fresh mozzarella into small pieces and scatter it over the gnocchi and sauce. Allow the mozzarella to melt into the dish, creating a creamy, cheesy texture.

Serve the Potato Gnocchi with Tomato Sauce and Mozzarella:

13. Plate the Gnocchi: Transfer the gnocchi and sauce to serving plates. Garnish with freshly grated Parmesan cheese and a few fresh basil leaves for added flavor and color.
14. Serve and Enjoy: Serve immediately while the gnocchi is hot and the mozzarella is gooey and melted.