Meanwhile, in a large deep non-stick skillet, melt the butter, sauté the leeks and garlic over medium-low heat until soft and golden, about 5 – 6 minutes. Add pureed butternut pumpkin, season with with salt and fresh cracked pepper and add a little of the reserved pasta water to thin out to your liking. Stir in parmesan cheese and sage and mix in pasta until well coated.