Place celeriac in a large saucepan. Cover with water, add 1 tablespoon lemon juice and season the water with salt. Bring to a boil. Adjust heat to maintain a gentle boil and cook, covered, for 15 minutes.
Meanwhile, peel pears and cut into 1-inch chunks. Add the pears to the pan; continue cooking until the celeriac and pears are very tender, about 10 minutes longer. Drain and puree in a food processor.
Return the puree to the pan. Stir in thyme and butter (or oil). Season with lemon juice, salt and pepper.
Make Ahead Tip: The puree can be kept warm in a water bath for up to 1 hour or it can be prepared ahead and stored, covered, in the refrigerator for up to 2 days. Reheat in a water bath or the microwave.
126 calories;2 g fat(1 g sat); 5 g fiber; 27 g carbohydrates; 3 g protein; 21 mcg folate; 3 mg cholesterol; 9 g sugars; 0 g added sugars; 74 IU vitamin A; 20 mg vitamin C; 89 mg calcium; 2 mg iron; 238 mg sodium; 645 mg potassium