Celery Root & Apple Purée

Put a large pot of water on to boil, and salt it well (about 1 Tbs. is good). Squeeze the lemon half into a large bowl of cold water. Peel the celery root and the apple. Cut each into 1-inch chunks and put them in the lemon water while working with the rest. Remove the celery root and apple chunks from the lemon water, put them in the boiling water, and boil until the celery root is tender, about 25 minutes. Drain and return the celery root and apple chunks to the empty pot. Shake the pot over low heat, drying the contents this way until steam no longer rises from the pot, 3 to 4 minutes. Transfer to a food processor. In a small saucepan over low heat, heat the cream and butter until the butter melts. Add 1 tsp. salt and then, with the food processor running, add the cream mixture. Process to a smooth purée, about 5 minutes. Season to taste with salt and nutmeg.