T.F.
That is what I am making!! You just reminded me to get it out of the freezer!
I do mine differently than a lot of recipies say. I do NOT put it in the oven at a high temp then turn oven off and don't open oven.
I DO..... use a fantastic meat thermometer that is nails down on target EVERYTIME. Now, I paid about $30 for my thermometer from my butcher which is where I purchase all my beef exclusively. It is all top quality from the best sources which also serve the high end 5 star restaurants in the Dallas area.
First...I let the roast stand at room temp about an hour before I put it in the oven. Mine has 3 ribs this Easter. I cover the roast in flour, salt, pepper for seasonings. Insert the termometer into the thickest part and place in over at about 350. My thermometer has the cord that comes out of the oven and I monitor it from outside the oven.
I slow roast until it is almost rare, then I blast it on about 450 to crisp the outside until the temp says rare. Then I remove it from the oven (It will continue to cook as it stands) wait about 10-15 minutes, carve and serve.
Of course timing is different depending on how you like yours cooked. We like ours rare.
I don't make the horseradish sauce... I have a specialty one from the deli that I bought. I also make an Au Jous sauce.
Enjoy