Gnocchi are feather light dumplings usually made solely from potatoes but can be made from many different starchy vegetables.
Cut squash in half, remove seeds, then toss with potatoes, olive oil, and salt. Put into a large pan with the rosemary sprig, cover, and roast at 400 degrees for one hour, or until tender and can be easily pierced with a paring knife. Be careful of escaping steam when checking to see if your squash and potatoes are cooked.
Bring a large pot of salted water to a rapid bowl. Get a bowl filled with ice cubes and water ready.
Remove squash and potato and place on your counter. Use a large tablespoon to scrape the meat out of the squash. Peel potatoes with a paring knife.
Put both squash and potatoes into a ricer. Mix with egg yolks, parmesan, salt, pepper, and nutmeg. If it needs a bit of seasoning do so to your personal tastes. Mix in flour and feel the dough, it should be solid enough to roll in strands but not too bouncy. It is hard to explain but at the end of the day, the goal is as little flour as possible.
Cut dough into eight equal pieces. Start working with one piece at a time. Roll dough out on floured surface to about 1/2-inch-thick rope. Cut each 'rope' into 15 pieces. Grab each piece and roll along the back of a fork dipped in flour, making ridges on 1 side. As soon as each 'rope' is done, drop into rapidly boiling water and cook until the gnocchi floats back to the top, about two minutes.
Using a spiral wire skimmer or slotted spoon, remove gnocchi from boiling water and drop into ice water. The gnocchi will go from soft to hard as they cool. Repeat with all the 'ropes'. Toss cooked gnocchi in oil and refrigerate till you are ready to eat.
Bring a large pot of salted water to a boil. Drop gnocchi in for two minutes.
While the gnocchi is reheating, brown butter in a heavy large saute pan over high till golden brown. Drop in sage leaves, chicken stock, lemon juice, and zest.,
Add gnocchi and Parmesan, adjust seasoning, toss well and serve.
A lot of recipes call for boiling potatoes but ignore them. Moisture is the enemy of gnocchi. Roast everything in the oven and it will keep your riced starches much dryer. Dryer equates to less flour. Less flour makes a lighter gnocchi.
After you create the dough you need to roll and cook as quickly as you can. The longer the dough sits the more it absorbs the flour and becomes sticky, requiring more flour.