Making the Filling

1. Prepare the Spinach: Place frozen spinach in a large skillet over medium-high heat, cover and cook for 5 minutes. Transfer to a sieve to drain and cool.
2. Combine Ingredients: Once cooled, squeeze out excess water from the spinach. In a food processor, combine spinach, ricotta, Parmigiano-Reggiano, nutmeg, salt, and pepper. Pulse until the mixture reaches a creamy consistency. Transfer to a bowl and set aside.

Creating the Tortelloni Dough

1. Mix and Knead: In a mixing bowl, combine flour, eggs, and salt. Using a dough hook attachment, knead on a medium setting for 5 minutes. Remove and knead by hand for an additional 2 minutes.
2. Resting the Dough: Shape the dough into an oval disc. Wrap in plastic wrap and let it rest for 30 minutes.
3. Rolling the Dough: Cut a 1-inch piece from the dough and flatten into a rectangle. Dust with flour on both sides. Using a pasta roller set to its widest setting, pass the dough through. Fold the dough in half and pass through again. Repeat. Gradually narrow the roller settings and pass the dough through, dusting with flour as needed. Continue until the dough is thin and smooth.
4. Shaping the Tortelloni: Lay the rolled dough on your work surface and let rest for 3 minutes. Use a 3-inch square cookie cutter to cut squares. Place a teaspoon of filling in the center of each square. Brush edges with water. Fold over to create a triangle, sealing the edges. Bring the two points of the triangle together and pinch to seal.

Cooking

1. Boil the Tortelloni: In a large pot of boiling salted water, cook tortelloni for 3-4 minutes, or until they float to the surface.

Making the Sauce

1. Combine and Melt: In a saucepan, melt butter with sage leaves over medium heat for 4 minutes.
2. Finish the Dish: Add the cooked tortelloni to the sauce, gently stirring to coat. Serve immediately, garnishing with additional Parmigiano, black pepper, and torn sage leaves.