Put the tomatoes and their juices in a food processor and pulse 4 or 5 times until the tomatoes are crushed but not completely pureed.

Remove the sausage from its casing and tear the sausage apart with your hands into coarse pieces. Heat 1 Tbs. of the olive oil in a 12-inch skillet over medium-high heat. Add the sausage and cook, further breaking it apart with a wooden spoon, until lightly browned and almost completely cooked through, 3 to 5 minutes. With a slotted spoon, transfer the sausage to a plate.

Add the remaining 2 Tbs. olive oil to the pan and then add the leek, parsley, thyme, marjoram, garlic, and salt. Cook, stirring, until the leek is soft and fragrant, about 3 minutes. Adjust the heat if the garlic or leek shows any sign of burning.

Add the tomatoes and reserved sausage and stir well to combine, scraping up any browned bits on the bottom of the pan. Bring to a vigorous simmer and then reduce the heat to medium-low. Cover the pan with the lid slightly ajar and simmer gently for 45 minutes. If the sauce is bubbling too fast, reduce the heat to low. Remove the lid and if the sauce seems watery, continue to simmer, stirring occasionally, until reduced to a thick sauce consistency. Adjust the seasoning to taste with salt and pepper.

Bring a large pot of well-salted water to a boil over high heat. To get the gnocchi into the boiling water, fold the parchment ends to form a chute and gently shake the gnocchi out, taking care not to clump them together as you drop them in. Give one gentle stir, wait until the gnocchi all float to the surface of the water, and then cook them for 1 minute. Carefully drain the gnocchi in a colander. Transfer them to the sauce and gently toss to coat. Serve immediately, topped with Parmigiano, if using.

Put the unpeeled potatoes in a large pot. Fill the pot with enough cold water to cover the potatoes by at least 2 inches and bring to a simmer over medium-high heat. Reduce the heat to medium, partially cover the pot, and simmer the potatoes until they are completely tender and easily pierced with a skewer, 30 to 35 minutes.

Drain the potatoes, let them cool just enough that you can handle them, and then peel them. Cut them in half crosswise and pass them through a ricer into a large bowl. Let cool until almost at room temperature, at least 20 minutes.

Lightly flour a work surface. In a small bowl, mix the flour with the salt. Add the egg to the potatoes and then add the flour mixture. Mix with your hands until the flour is moistened and the dough starts to clump together; the dough will still be a bit crumbly at this point. Gather the dough together and press it against the bottom of the bowl until you have a uniform mass. Transfer it to the floured surface and wash your hands.

Knead gently until the flour is fully incorporated and the dough is soft, smooth, and a little sticky, 30 seconds to 1 minute. (Don't overmix it, or the gnocchi will be tough; the dough should feel very delicate.) Move the dough to one side, making sure the surface underneath it is well floured. Cover it with a clean kitchen towel.

Cover two large rimmed baking sheets with parchment and sprinkle lightly with flour.

Remove any lingering bits of dough from your work surface and lightly reflour the surface. Tear off a piece of dough about the size of a large lemon and put the towel back on the rest of the dough so it doesn't dry out.

With the palms of both hands, roll the dough piece on the floured surface into a rope about 3/4 inch in diameter.

With a sharp knife or a bench knife, cut the rope crosswise every 3/4 inch to make roughly 3/4-inch-square gnocchi. Arrange the gnocchi in a single layer on the parchment-covered baking sheets, making sure they don¿t touch. Repeat until you run out of dough, reflouring the work surface as needed. When all the gnocchi have been cut and spread out on the baking sheets, sprinkle them with a little more flour.

If you're going to use the gnocchi within 2 to 3 hours, they can sit out on the counter.