Step 1: Bake potatoes at 400°F (200°C) for 45-60 minutes until tender. Let cool slightly, then peel and rice or mash until smooth.
Step 2: Spread mashed potatoes on a baking sheet to cool completely to room temperature (about 30 minutes).
Step 3: On a clean work surface, make a mound with cooled potatoes. Create a well in the center and add egg, salt, pepper, and nutmeg.
Step 4: Gradually incorporate flour into potato mixture, kneading gently until a soft dough forms (about 3-4 minutes). Don’t overwork.
Step 5: Divide dough into 4 portions. Roll each into a 3/4-inch thick rope on a lightly floured surface.
Step 6: Cut ropes into 1-inch pieces. Optionally, roll each piece over a gnocchi board or fork tines for ridges.
Step 7: Bring a large pot of salted water to a boil. Cook gnocchi in batches until they float (2-3 minutes). Remove with slotted spoon.
Step 8: While gnocchi cook, melt butter in a large skillet over medium heat. Add garlic and cook 1 minute until fragrant.
Step 9: Add crushed tomatoes to skillet, season with salt and pepper, and simmer for 10 minutes.
Step 10: Add cooked gnocchi to sauce, gently toss to coat. Stir in fresh basil.
Step 11: Serve immediately topped with grated Parmesan cheese.