01 - Preheat oven to 400°F (200°C). Halve the acorn squash, remove seeds, brush with olive oil, and place cut side down on a baking sheet. Roast for 30–35 minutes, or until tender. Allow to cool, then scoop out the flesh and mash until smooth.
02 - In a mixing bowl, combine the mashed acorn squash, ricotta cheese, Parmesan cheese, nutmeg, salt, and black pepper. Mix thoroughly and set aside.
03 - On a clean work surface, form a mound with the flour and create a well in the center. Add the eggs, salt, and olive oil into the well. Gradually incorporate the flour with a fork, then knead by hand for 10 minutes until the dough is smooth and elastic. Wrap tightly in plastic wrap and allow to rest for 30 minutes.
04 - Divide the rested dough in half. Roll out each half on a lightly floured surface or using a pasta machine into thin sheets, approximately 1/16 inch (1.5 mm) thick. Place teaspoonfuls of the filling about 1 1/2 inches (4 cm) apart along one sheet of dough. Lightly brush the areas around the filling with water. Place the second sheet of dough on top, pressing gently to seal and remove any air pockets. Cut the dough into individual ravioli squares.
05 - Bring a large pot of generously salted water to a rolling boil.
06 - Meanwhile, in a skillet set over medium heat, melt the unsalted butter. Add the chopped hazelnuts and fresh sage leaves. Cook, swirling the skillet occasionally, until the butter turns a golden brown color and releases a nutty aroma, approximately 3–4 minutes. Remove the skillet from the heat and stir in the lemon zest. Season with salt and pepper to taste.
07 - Gently place the ravioli into the boiling water in batches. Cook for 2–3 minutes, or until they float to the surface.
08 - Carefully transfer the cooked ravioli to serving plates using a slotted spoon. Spoon the hazelnut brown butter sauce over the ravioli. Garnish with shaved Parmesan cheese and additional chopped toasted hazelnuts, if desired. Serve immediately.