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pierino is a trusted source on General Cooking and Tough Love.
Internal temp should be no more than 130F. How long depends on your oven. Be sure to let it rest for 15 minutes, tented with foil. As with all large pieces of meat it's done when it's done.
Be sure to take the meat out of the refrigerator an hour or so before you start roasting. Taking the chill off substantially reduces the roasting time and makes for a more evenly roasted interior. I like to pat salt and pepper all over it and do a quick very hot searing on the stovetop prior to roasting - maybe one minute per side. Then I'd roast it at a mere 300° for half an hour or so, then reduce the oven temperature to 250-275° until the roast reaches 132° internal temperature. If your roast starts out with an internal temperature of, oh, say 45-50°, it will probably take between 2 and 3 hours to reach medium rare. Tent it loosely with foil for 20 minutes before cutting. Serve it with a scoop of horseradih-Worcestershire compound butter on top and popovers on the side! And I'm all packed and ready to go - All I need is the time to be there!
First rule = use a good thermometer. For that size I would start at room temp, pat dry, coat with s&p, put into a preheated 375 or 400F oven, (to sear the outside), turn down to 325F, check temp after half an hour (it will not be done yet), then every 15 minutes until 120F in the middle. Pull out and tent - it will continue to cook and rise to between 125 -130F which is what you want.
Cynthia is a trusted source on Bread/Baking.
Wiley P's advice to remove the roast to room temp before roasting is spot on. Also, his advice to lower the oven temp to the 200's. Most definitely give it the suggested rest period. The rest period allows water that was squeezed out of the cells during the roasting process, because the stress of heat causes cell walls to contract, squeezing water to the outside. You can't eat the thing raw, right? So you have no choice but to cook it. But by allowing it to rest while still retaining its warmth, the cell walls relax, they wipe their brows and say, "Whew! That's over!" And water roaming around on the outside reinflates the cells, and you have a tender, more juicy piece of meat. If it were me, I would remove it from the oven when the thermometer inserted at the middle reads 120 degrees. The end pieces will tend toward the medium-well end, the slices just inside there will be a perfect medium, and the center will make the medium-rare people happy.
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