Step 1

In a large saucepan, melt butter over medium-high heat. Add onion and garlic and cook, stirring often, until softened and translucent, about 5 minutes; lower heat as necessary to prevent browning.

Step 2

Add cauliflower, cream and/or stock, and thyme. Cover, bring to a simmer, and cook, adjusting heat to maintain simmer, until cauliflower is tender, 5 to 6 minutes. Uncover and continue to simmer, stirring occasionally, until liquid is reduced by about three-quarters, about 20 minutes. Discard thyme sprig.

Step 3

Using a blender or immersion blender, blend cauliflower and liquid to form a very smooth purée. Season with salt. (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 whisking in liquid, such as stock, cream, or water.) Serve warm.