1. Put celery root, thyme sprig, and garlic into a small saucepan. Cover with cold water by 1 inch. Bring to a boil. Reduce heat; add 2 teaspoons salt and the potato. Simmer until vegetables are very soft, about 10 minutes.

2. Drain; discard thyme sprig. Transfer vegetable mixture to a food processor. Add milk and butter, and process until just pureed. Season with salt and pepper. Garnish with thyme. Serve with a pat of butter.