1 pound whole-milk ricotta cheese
1/2 pound Swiss chard leaves (from about 1 pound Swiss chard, stemmed), rinsed well under cold water (spinach leaves can be substituted for Swiss chard leaves)
2 large egg yolks
1 teaspoon coarse sea salt, preferably gray salt
1/2 teaspoon packed freshly grated nutmeg
1/4 teaspoon freshly ground black pepper
Place the ricotta in a cheesecloth-lined colander set over a bowl. Cover the ricotta with plastic wrap, set a heavy object (such as a small cast iron pan topped with canned goods) over the ricotta to help weigh it down. Let it drain for 30 minutes.
Bring a large pot of water to a boil. Season the water with enough salt so that it tastes like the ocean. Set up an ice bath (a large bowl of water and ice cubes).
Add the Swiss chard leaves to the boiling salted water and cook for 3 minutes. Drain, then plunge the chard into the ice water bath to stop the cooking process, leaving the chard in the cold water for 1 minute. Drain again, squeezing out as much water as possible. Wrap the chard in paper towels to remove excess moisture.
Coarsely chop the chard and transfer it to a food processor along with the drained ricotta. Pulse to combine, about 20 pulses. Add the egg yolks, salt, pepper and nutmeg and pulse until combined and the chard is finely chopped.
Taste the filling and make sure it is well seasoned with salt and pepper. Spoon it into a pastry bag fitted with a 1/2-inch round tip and refrigerate until ready to use. Note: if using a disposable pastry bag, you can trim the tip with scissors to create a 1/2-inch wide opening.