Preheat the oven to 190°C (170°C fan). Halve the butternut squash lengthways, scoop out the seeds, and score the flesh lightly with a knife. Place it flesh-side up in a baking tray.
Drizzle the cut garlic head with olive oil, wrap it in foil, and add it to the tray.
Roast for 45–50 minutes, until the squash is very tender. After 25-30 mins, add the onion to the tray, tossing in the oil on the tray.
Scoop out the roasted squash flesh and place it in a blender with the roasted onions. Squeeze in the soft roasted garlic cloves.
Add the cream and blend until silky smooth, adding a splash of pasta water if needed. The sauce should be thick but pourable. It must be silky smooth so if it's not, keep adding liquid and blending until it's velvety. Season well with salt and pepper.
Bring a large pot of salted water to the boil and cook the pasta until al dente.
In a separate large frying pan, heat over medium heat and melt the butter. Add the sage leaves and fry until the butter foams and turns golden brown.
Once golden, remove the crispy sage leaves from the pan and set aside on a plate with a sheet of kitchen paper to absorb the excess butter.
Pour half the brown butter into a small bowl for later and leave the rest in the pan. Add the drained pasta to the pan with the brown butter and toss well.
Add the butternut squash sauce, most of the Parmesan, and toss to coat the pasta well. You may need to add a splash of pasta water to loosen it.
Divide the pasta between bowls. Top with crispy sage leaves, more grated Parmesan, and drizzle over the reserved brown butter.
The video below will help with this recipe.