We've made this, but it uses prepared horseradish, not fresh, so I'd either cook the fresh or probably start out with about 1/3 cup instead of a whole cup then add as needed, fine grate, you don't want big pieces of horseradish in your sauce.
1 cup well-drained prepared horseradish
1/2 cup sour cream
1/2 cup mayonnaise
2 tablespoons fresh squeezed lemon juice
1 teaspoon kosher salt
1/4 teaspoon freshly ground black pepper
2 teaspoons Worcestershire sauce
Few drops Tabasco sauce
Mix all ingredients well, refrigerate and allow to meld for several hours or overnight.
We've also made this.
3/4 cup fresh finely grated horseradish
3 tablespoons finely minced shallots
2 teaspoons finely minced garlic
2 teaspoons Dijon mustard
1-1/2 cups heavy cream
salt and white pepper to taste
In a saucepan, combine all ingredients. Bring to a boil, whisking gently. Reduce heat and simmer for about 12 minutes, stirring occasionally, until thick enough to coat the back of a spoon. Adjust seasoning. Remove from heat and keep warm until serving.
You can also whip some heavy cream just until soft peaks form, stir in some finely grated horseradish to taste, add salt and pepper and a touch of lemon juice. There used to be a restaurant we went to that served this with their deep fried mushrooms and zucchini chips. Yummy!