1(28-ounce) can whole peeled tomatoes(preferably San Marzano)
1medium onionpeeled and cut in half
10ouncesspinach leavesno stems, stemmed and rinsed
4cupsricotta impastata or drained whole-milk ricotta
1large egglightly beaten
1 ⅓cupstipo 00 or all-purpose flourplus more for tossing gnudi
Freshly grated nutmeg
Kosher salt and freshly ground pepper
To make the sauce: Add tomatoes with their juices, butter and onion to a medium saucepan.
Uncovered, over medium heat bring the sauce to a simmer then adjust the heat to keep the sauce at a slow, steady simmer for about 45 minutes, or until droplets of fat float free of the tomatoes. Stir occasionally, breaking up the tomatoes with a spoon against the side of the pot or gently mashing them with a potato masher. Discard onion, and season with salt if needed. Puree in a blender if you prefer a smoother sauce.
To make the gnudi: Bring a large pot of water to a boil. Add the spinach leaves and blanch for 1 minute. Remove, let cool, squeeze dry, and finely chop. Measure out 7 ounces of spinach (about ¾ cup packed) and put it in a mixing bowl. Mix in ricotta, egg, the 1 ⅓ cups flour, and nutmeg. Season with salt and pepper and mix thoroughly but gently.
Put 1 to 2 cups more flour in a pie pan or shallow bowl. Put the gnudi mixture in a pastry bag fitted with a ½-inch plain tip or a ziplock bag with a ½-inch hole snipped from a corner. Pipe ½-inch-diameter balls of the dough into the flour a few at a time. Use the edge of a knife or fork to cut the dough as needed while piping. Gently roll into dumplings in the flour and transfer to a baking sheet coated generously with flour.
Bring a large pot of salted water to a boil. Drop in the gnudi, quickly return to a boil, and cook until the gnudi float, about 2 minutes, then cook 20 seconds longer. Each dumpling should feel soft, delicate, and slightly springy when pressed with a fingertip, and have a creamy center when bitten. Drain the pasta. Serve with the sauce.
You can also use tomato puree or diced tomatoes for the sauce, but it will have slightly less texture (especially with the puree). It will also need less simmering time since the tomatoes are already broken down.