Roast the squash: Preheat the oven too 400°F. Slice squash in half lengthwise, scoop out the seeds. Brush cut sides with olive oil, sprinkle with salt and pepper. Place cut side down on a parchment lined sheet pan. If using rosemary or sage, tuck the sprigs underneath. Roast 40 -50 minutes, until very tender. Cool slightly, then scoop out flesh.
Sauté aromatics: In a skillet, heat a drizzle of olive oil. Cook onion until soft (about 5 minutes), then stir in garlic for 1 minute.
Blend the sauce: Add roasted squash, onion/garlic mixture, and broth to blender (or use and immersion blender in the pan). Blend until smooth and creamy.
Finish the sauce: Return sauce to the skillet. Stir in cream, Parmesan, and nutmeg. Simmer gently for 5 minutes, stirring until warmed through. Taste and adjust seasoning.
Cook the ravioli: Meanwhile, cook ravioli according to package directions. Drain and toss with the butternut squash sauce.
Serve: Plate the ravioli, sprinkle with extra Parmesan, and garnish with a little fresh thyme or cracked pepper if you like.
