Place the ricotta in a fine sieve over a bowl. Cover with plastic wrap and refrigerate for at least 8 hours or overnight.

 In a large skillet, heat the butter over medium high heat until foaming. Add the leeks and cook, stirring, until wilted, about 4 minutes. Stir in the spinach, season lightly with salt and pepper, and cook stirring, until tender but bright green, about 5 minutes. Scrape the spinach mixture into a sieve placed over a plate. Press lightly to remove excess liquid. Cool completely.

 In a medium size bowl, combine the drained spinach mixture and the drained ricotta. Add the egg and marjoram, season with salt and pepper to taste, and stir until well blended. 

 While you prepare the gnocchi, bring the water to a boil in a fish poacher. Cut three 12 x 18 inch pieces of cheesecloth and stock them one atop the other.

 On a well floured work surface, roll the dough with a floured rolling pin out to a 12 x 15 inch rectangle, lifting the dough once or twice with the help of a large metal spatula or dough scraper to floor the surface underneath. Spread the spinach mixture in an even layer over the entire surface of the dough rectangle.

 Starting from one of the rectangle’s longer sides and working with the metal spatula or dough scraper to help you, roll the dough into a tight cylinder as you would a jelly roll. Center the roll over the cheesecloth. Wrap the roll tightly in the cloth and tie the ends securely with a kitchen string. Carefully lower the roll into the boiling water. Adjust the level of heat to a gentle boil and cook until a knife inserted comes out clean, about 1 hour. Remove the strucolo from the water, drain, and allow standing for about 15 minutes.

Meanwhile, preheat the oven to about 375 degrees and prepare the sauce. In a small skillet, melt the butter over very low heat, swirling the skillet constantly until the butter is melted and creamy but not separated. Add the sage and cook for 1 minute. Keep the sauce in a warm spot.

 Unwrap the strucolo and place on a cutting board. With a serrated knife, using a gentle back and forth motion cut the roll into ½ inch thick slices. Arrange the slices side by side in one large (about 18 x 12-inch) baking dish or two small (10-inch round or 8-inch square) buttered baking dishes. Drizzle the sage sauce and leaves over the slices. Bake for 20 minutes, then sprinkle with half of the grated cheese and bake until the top is golden brown and crisp, about 10 minutes. Serve with remaining grated cheese.

Note: A fish poacher—a long, oval pan with a perforated removable rack—is the ideal vessel for cooking the strucolo. If you don’t have one, use your widest, deepest pot or roasting pan, but measure it first to make sure it is wide enough to fit the finished roll (about 16 inches long) lengthwise and deep enough to keep the roll submerged in water during cooking. Make these arrangements in advance; once the gnocchi dough is rolled out and filled, the roll must be poached immediately.

 To prepare the dish entirely in advance: Cook the strucolo as described above. Unwrap the cooling strucolo after 15 minutes and let it stand until completely cool. Cut the roll into ½ inch thick slices and arrange slices side by side in a buttered baking pan. Drizzle the sage sauce and leaves over the slices and cover the pan with aluminum foil. Refrigerate for up to one day. Let the pan of strucolo stand at room temperature for 1 hour. Preheat the oven to 350 degrees F. Bake for 20 minutes, uncover the pan, sprinkle the slices with grated cheese, and bake until the strucolo is golden brown and crisp on the surface, about 10 minutes.