While the potatoes are crisping up, make the horseradish cream. In a bowl, combine the mayonnaise, horseradish, salt, pepper, and lemon juice. Mix until combined. If you want the sauce to be a thinner consistency, add a bit of water. Taste for seasoning. Depending on the type of horseradish you get will determine the potency. Feel free to add more or less to your liking.