Slice both ends of the butternut squash. Peel the outer skin of the squash. We do this by standing the squash on the flat bottom then running a knife down the sides.
Cut the squash in half lengthwise and remove the seeds. Cut squash into 2-inch cubes and place in a large mixing bowl.
Add 3 tablespoons of the olive oil and toss with the cubes. Sprinkle with garlic salt and black pepper and toss to distribute seasoning.
Move squash to a baking sheet and set aside.
Add shallots to a small mixing bowl and pour in the remaining olive oil. Make a bowl out of foil and add the shallots to it.
Place the foil bowl of shallots on the baking sheet with the squash and roast in the preheated oven for 30-35 minutes.
Remove from oven and allow to cool for 10 minutes.
Add squash, shallots, garlic, chicken broth, and half of the half and half to the mini food chopper and blend until smooth. The mixture will be fairly thick.
Allow to cool before storing. Store sauce in a jar or container with a lid in the refrigerator for up to a week.