Combine all ingredients in a medium bowl until evenly distributed. Store in a tightly sealed container at room temperature.
For the Roast
Remove roast from the refrigerator, loosely wrapped, 3 hours before cooking. Roasts should always be brought close to room temperature first, before they go in the oven. (This helps them cook more evenly.)
Cookbooks often call for the excess fat to be removed. By "excess" fat they mean any fat more than an inch thick. The fat is what provides the flavor and what you are paying for with prime rib, so you want to leave it on. Your butcher should have removed any excess fat.
If your butcher hasn't already done so, cut the bones away from the roast and tie them back on to the roast with kitchen string. This will make it much easier to carve the roast, while still allowing you to stand the roast on the rib bones while cooking.
Preheat your oven to 500°F, or the highest it will go (our oven only goes up to 450°F). Generously sprinkle salt and pepper all over the roast. Alternately, you can replace the salt and pepper with a dry rub (such as the one below) of your choice.
Insert a meat thermometer into the thickest part of the roast, making sure it doesn't touch a bone. (Some meat thermometers require that you poke a hole first with a skewer, and then insert the thermometer.) Place the roast, fat side up, rib side down in a roasting pan in the oven.
After 15 minutes on 500°F, reduce the heat to 325°F. To figure out the total cooking time, allow about 13 to 15 minutes per pound for rare and 15 to 17 minutes per pound for medium rare. The actual cooking time will depend on the shape of the roast and your particular oven. A flatter roast will cook more quickly than a thicker one. So make sure to use a meat thermometer. This is not a roast to "wing it". Err on the rare side.
Roast in oven until thermometer registers 115°F to 120°F for rare, or 125°F to 130°F for medium.
Check the temperature of the roast using a meat thermometer a half hour before you expect the roast to be done. For example, with a 10 pound roast, you would expect 2-½ hours of total cooking time (15 minutes at 500°F and 2-¼ hours at 325°F). In this case, check after 2 hours of total cooking time, or 1-¾ hours after you lowered the oven temp to 325°F.
Once the roast has reached the desired internal temperature, remove it from oven and let rest 20 minutes, covered with aluminum foil, before carving. The roast will continue to cook while it is resting.
With a knife or scissors, cut the strings, which attach the meat to the bones. Remove the bones (save for making stock for soup. Then, using a sharp carving knife, slice meat across the grain for serving, making the slices about ¼ to ½ inch thick.
For the Yorkshire Pudding
Preheat the oven to 450°F.
Sift together the flour and salt in a bowl. In another bowl, beat together the eggs and milk until light and foamy. Stir in the dry ingredients just until incorporated.
Pour the drippings into a 9-inch pie pan, cast iron skillet, or square baking dish. Put the pan in oven and get the drippings smoking hot. Carefully take the pan out of the oven and pour in the batter. Put the pan back in oven and cook until puffed and dry, 15 to 20 minutes.
For the Horseradish Cream Sauce
Beat the heavy cream until stiff. Slowly add the horseradish and lemon juice and continue to beat together. Taste and adjust seasoning if needed. Garnish top with chives. Serve with roasted beef.
For the Sour Cream Horseradish Sauce
In a medium bowl, combine all the ingredients and whisk until smooth. Taste and adjust seasonings. Cover and refrigerate until ready to use. Will hold in the refrigerator up to 2 days.
For the Bernaise Sauce
Simmer the shallots in vinegar and white wine until liquid has been reduced by about two-thirds. Remove from heat and cool.
Place this mixture in blender and add egg yolks, tarragon, peppercorns, salt, and dry mustard. Blend at high speed for no more than 10 seconds.
Add the warm melted butter, cayenne, and a squeeze of lemon juice (optional). Quickly turn on blender again and let it rotate until sauce thickens. Should it become too thick, add a bit of hot water and blend again.
This sauce should be made at the last minute so that it will be warm when served (but it may be kept warm in a double boiler).