1 pound fresh broccoli rabe, trimmed
1 pound waxy golden potatoes, such as Yukon Gold, peeled and cut into 1/2-inch dice
1 1/2 cups fresh ricotta, drained
1/2 cup freshly grated Parmigiano-Reggiano, plus more for serving
1/4 cup extra virgin olive oil
Freshly ground black pepper, to taste
1 cup semolina flour, for dusting
12 tablespoons (1 1/2 sticks) unsalted butter
15 fresh sage leaves
Note: Save any extra ravioli filling and use it to make a frittata.
Bring 3 quarts of water to a boil in a large pot. Set up an ice bath nearby.
When the water is boiling, add 1 tablespoon salt. Then add the broccoli rabe and cook until very tender, about 6 minutes. Using a spider or a slotted spoon, transfer the rabe to the ice bath.
Add the potatoes to the boiling water and cook until tender, 8 to 10 minutes. Drain, and pass through a ricer or mash with a potato masher.
Once the rabe has cooled, drain and pat it dry. Chop the rabe until it is almost a paste (you can do this in the food processor).
In a medium bowl, combine the rabe, potatoes, ricotta, Parmigiano, olive oil, and salt and pepper to taste. Stir well to combine.
Roll out the pasta dough to the thinnest setting on a pasta machine, using a good dusting of the semolina between the resting sheets to maintain a good separation. Cut the pasta sheets into 4-inch squares. Place a scant tablespoon of the ricotta filling on on e half of each square, and then fold it over like a book to enclose the filling, forming a rectangle. Press the edges of the ravioli with your fingers to seal them. (The ravioli can now be frozen between sheets of parchment, sprinkled with semolina, until ready to cook.)
Bring 8 quarts of water to a boil.
While the water is heated, heat the butter in a 14-inch sauté pan over high heat until the foam subsides and it begins to turn a light brown color. Add the sage and remove the pan from the heat.
Add 2 tablespoons salt to the boiling water. Then add the ravioli, and cook until they are tender and cooked through (you should taste one to test it), about 3 minutes (a little more if frozen). Reserving 3 tablespoons of the cooking water, drain the ravioli. Add them to the pan containing the butter and sage. Add the reserved cooking water, and toss gently over high heat for 1 minute.
Pour the hot ravioli onto a warmed platter, and serve immediately with Parmigiano grated over the top.