Roast the squash. Preheat the oven to 400°F. Slice the butternut squash in half lengthwise and scoop out the seeds. Brush the cut sides with olive oil and season with salt and black pepper.
Place the squash cut-side down on a parchment-lined baking sheet. Tuck the rosemary or sage underneath the squash, if using. Roast for 40 to 50 minutes, or until the squash is very tender when pierced with a fork.
Let the squash cool just enough to handle, then scoop the flesh from the skin and set aside.
Sauté the aromatics. Heat a drizzle of olive oil in a large skillet over medium heat. Add the onion and cook for about 5 minutes, until softened. Add the garlic and cook for about 30 seconds, just until fragrant.
Blend the sauce. Add the roasted squash, sautéed onion and garlic, and broth to a blender. Blend until completely smooth. You can also combine everything in a deep pan and use an immersion blender.
Finish the sauce. Pour the sauce into a large skillet over medium-low heat. Stir in the heavy cream, Parmesan and nutmeg. Simmer gently for about 5 minutes, stirring occasionally. Taste and adjust the salt and black pepper as needed.
Cook the ravioli. Meanwhile, prepare the ravioli according to the package directions. Before draining, reserve about ½ cup of the pasta water.
Drain the ravioli and gently toss it with the warm butternut squash sauce. If the sauce is thicker than you'd like, stir in a little reserved pasta water until it reaches your preferred consistency.
Serve immediately with extra Parmesan and freshly cracked black pepper.