For the dip: Peel and roughly chop your horseradish root. Puree it in your food processor on high with the lemon juice, buttermilk, Dijon, and egg yolk. If the mixture is not a wet puree, add a few drops of water. Turn the machine to low and drizzle the oil until an emulsion is formed. Taste and adjust with salt, white pepper, and lemon juice, if necessary. Chill until ready to serve.