Steam or boil the butternut squash until fork-tender. Drain well.
Transfer the cooked butternut squash to a blender or food processor and add all remaining ingredients. Blend until a mostly smooth sauce.
Transfer the sauce to a saucepan over medium-low heat. Cover and bring up to a simmer.
Stirring occasionally, let simmer 15 minutes. It is best to remove from the heat every few minutes to stir and then re-cover before returning to the heat, as the sauce will splatter.
Rinse out blender or food processor before transferring the cooked sauce back into it. Blend until entirely smooth. Store covered and refrigerated for up to 5 days.
