Well, I have tried this recipe two ways now, one with the food processor and one with the food mill, and finally, I know what these cookbook authors are talking about. The food processor potatoes were sticky and viscous, and the food mill ones had a lighter mouthfeel and were more delicate and precious in a way that Alice Waters might approve of. Though I will serve the food mill ones to people I want to impress, I secretly liked the gluey stuff. That version was more like dessert--apple-potato-celeriac dulce de leche or something.
(Apologies for the quarter-chicken sitting next to the mash.)
4 T butter, separated
1 head of celeriac (celery root), 1/2-inch dice (peeled!)
1 granny smith apple, 1/2 inch dice (peeled!)
2 medium Yukon gold potatoes, 1/2-inch dice (peeled!)
1 t salt
1/2 t pepper
1/2 cup water
1/4 c heavy cream
1/4 c milk
squeeze of lemon
In medium skillet melt 2 tablespoons of the butter, and then add the celeriac, apples, potatoes, salt and pepper. Cook for 5 minutes or so, until it just begins to soften. Add the water (you could also use wine, or cider), cover, and cook for 25-40 minutes, stirring often, until everything is very soft. Add more liquid if it begins to burn. Transfer the mixture to a food processor or put it through a food mill and puree. Heat the cream, milk, and rest of the butter in a small saucepan. Return the potato-apple mixture to the original pan. When the cream/milk mixture is hot, stir it into the puree. Add lemon, taste for seasoning, serve hot.