Step 1

Adjust oven rack to middle position and preheat oven to 500°F (260°C). Place cauliflower on a foil-lined rimmed baking sheet, toss with olive oil, and season with salt and pepper. Roast, flipping cauliflower with a thin metal spatula halfway through roasting, until cauliflower is tender and deeply browned on both sides, about 20 minutes total.

Step 2

In a large saucepan, melt butter over medium-high heat. Add onion, garlic, and cayenne and cook, stirring often, until softened and just starting to turn golden, about 4 minutes.

Step 3

Add cauliflower, cream and/or stock, and thyme. Bring to a simmer, then cook, adjusting heat to maintain simmer, until cauliflower has softened in the liquid, about 5 minutes. Discard thyme sprig.

Step 4

Using a blender or immersion blender, blend cauliflower and liquid to form a very smooth purée. Season with salt and pepper. (You can adjust purée consistency as needed: Thicken by cooking down further while stirring often over low heat in a wide nonstick skillet, or thin by adding liquid, such as stock, cream, or water.) Serve warm.