Add the frozen spinach in a large skillet over medium-high heat, cover and cook for 5 minutes, transfer into a large sieve to drain while it cools.
When cooled, squeeze all the excess water from the spinach.
Put the spinach in a food processor along with the ricotta, Parmigiano-Reggiano, nutmeg, salt and pepper.
Pulse until it becomes a creamy consistency. Transfer to a bowl and set aside.
FOR THE TORTELLONI DOUGH
Add flour, eggs, and salt in the mixing bowl and attach the dough hook. Turn to speed 2 and knead for 5 minutes. Remove the dough and knead by hand for 2 minutes. Shape into an oval disc and let it rest for 30 minutes wrapped in plastic wrap.
Cut the dough into a 1-inch piece and flatten into a rectangular shape, add a small amount of flour to both sides.
Attach the pasta sheet roller to your stand mixer and set it to #1. Turn on the stand mixer to speed 2 and run the pasta dough through the roller. While on #1, fold the dough in half and run it through again. Do this twice.
Adding a little bit of flour on each side of the dough again, change setting to #2 and pass the pasta dough through the sheet roller. Do this twice and then twice each on #3 and then once on #4 #5 #6.
Once again, add flour to each side of your long pasta sheet and set on your working table.
Let them rest for 3 minutes.
Cut into squares with a 3-inch square cookie cutter.
Put one heaping teaspoon of filling on each square, brush water all around the edges.
Pick up the square and stretch the sides do not stretch the corner.
Line up two opposite corners and pinch to seal to form a triangle.
Take two of the opposite sides of the triangle and fold to the front and pinch together.
In a large pot of boiling, salted water, add the pasta and cook for 3-4 minutes.
FOR THE BUTTER SAGE SAUCE
In a medium saucepan melt the butter and sage and melt over medium heat for 4 minutes.