Place the Russet Potatoes whole (with skins on) in a large pot, cover with cold water, add a pinch of Coarse Salt, and simmer until tender when pierced with a fork, about 30–40 minutes depending on size.
Drain and let cool just enough to handle; peel the potatoes and pass them through a ricer or mash very finely while still warm to avoid a gluey dough.
Spread the riced potatoes on a work surface to cool slightly, then sprinkle with 00 Flour and a pinch of Fine Salt. Make a well, add the Large Egg, and gently mix with your hands until a soft, cohesive dough forms — work quickly and add only enough flour to keep it from sticking.
Divide the dough into portions, roll each into ropes about 1.5 cm (½ inch) thick, and cut into 2 cm pieces. If you like, press each piece lightly with a fork to create ridges for sauce to cling to.
Bring a large pot of water to a rolling boil, salt generously with Coarse Salt, and cook the gnocchi in batches; they are done when they float to the surface, about 1–2 minutes. Use a slotted spoon to transfer them to a bowl while you prepare the sauce.
For the sauce, warm Extra-Virgin Olive Oil in a skillet over medium heat, add gently crushed Garlic Cloves and sweat until fragrant but not browned, then pour in the Passata di Pomodoro.
Simmer the sauce for 10–15 minutes until slightly reduced; season with Fine Salt to taste and stir in most of the Fresh Basil Leaves, reserving a few for garnish.
Combine the drained gnocchi with the tomato sauce in an ovenproof dish, tear or scatter pieces of Fior di Latte Mozzarella over the top, and grate a generous amount of Parmigiano Reggiano before placing under a hot broiler or in a 200°C (400°F) oven until the cheese is bubbly and lightly golden, about 8–10 minutes.
Remove from the oven, scatter the remaining Fresh Basil Leaves over the top, let rest a couple of minutes, then serve immediately for a comforting, glossy finish.
