01 - Preheat oven to 400°F. 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 surface, form a mound with the all-purpose flour and create a well in the center. Add the eggs, fine sea salt, and olive oil to the well. Gradually incorporate the flour using a fork, then knead the dough by hand for 10 minutes until it becomes smooth and elastic. Wrap the dough in plastic wrap and let it rest for 30 minutes.
04 - Divide the rested dough in half. On a lightly floured surface, roll out each half into thin sheets, approximately 1/16 inch (1.5 mm) thick, using a rolling pin or a pasta machine.
05 - Place teaspoon-sized portions of the filling about 1 1/2 inches apart along one sheet of pasta. Lightly brush the areas around the filling with water. Place the second pasta sheet on top. Press firmly to seal the edges and cut the ravioli into desired square shapes.
06 - Bring a large pot of salted water to a rolling boil.
07 - While the water heats, melt the unsalted butter in a skillet over medium heat. Add the toasted hazelnuts and fresh sage leaves. Cook, swirling the skillet occasionally, until the butter turns a golden brown color and emits 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.
08 - Gently boil the ravioli in batches for 2-3 minutes, or until they float to the surface.
09 - Transfer the cooked ravioli to serving plates. Spoon the hazelnut brown butter sauce generously over the top. Garnish with shaved Parmesan and additional chopped toasted hazelnuts, if desired. Serve immediately.