This is from my condiments cookbook, Homemade Condiments by Jessica Harlan:
Prepared Horseradish Sauce
Seek out fresh horseradish root at your local supermarket or specialty grocery store and youll be able to make your own fresh horseradish, a spicy ingredient to stir into everything from Bloody Marys to steak sauce. Look for horseradish root that appears to have been recently cut: the center should be white (rather than yellowish) and moist. Its very pungent, so I recommend using food-handling gloves to work with it.
Makes about 8 ounces
1/2 pound fresh horseradish root
1/4 cup white vinegar, more if needed
1 teaspoon kosher salt
Use a paring knife or vegetable peeler to peel the hairy brown skin from the horseradish. Grate the root into a bowl with a fine box grater or rasp grater. Transfer the grated horseradish to the bowl of a food processor. Add about half the vinegar and half the salt and pulse to combine and further grind the horseradish. Add more vinegar until the mixture is fairly smooth and spreadable but not watery. Season to taste with additional salt. Keep covered in the refrigerator for up to 1 month (use a sterilized glass jar, as a plastic container will absorb the flavor and scent).
Use It For
Season a Bloody Mary.
Add a kick to tuna or chicken salad.
Mix with mayonnaise and spread on rye bread for a roast beef sandwich.
Stir into mashed potatoes.
Mix with ketchup for a seafood dipping sauce

Cher