Preheat the oven to 400°F. Cut the cauliflower into small florets (you'll end up with about 5 cups). In a medium bowl, toss half of the cauliflower with the oil, 1/2 teaspoon salt and 1/4 teaspoon pepper until coated. Arrange in a single layer on a baking sheet and roast, stirring occasionally, until golden brown and tender, about 30 minutes.

Place the remaining florets in a medium saucepan with the half-and-half, butter, and 1/2 teaspoon salt and 1/4 teaspoon pepper. Simmer over medium heat until the cauliflower is very tender, about 20 minutes. Drain, reserving about 1 cup of the cooking liquid.

Place the simmered cauliflower and garlic in a blender. Process into a very smooth puree, adding just enough of the reserved cooking liquid. Transfer to a small heavy-bottomed saucepan. Chop the roasted cauliflower and fold it into the puree. Adjust the seasonings and keep warm over low heat until ready to use, or store in an airtight container in the refrigerator for up to 4 days. Reheat in a saucepan over low heat.