Remove the roast from refrigerator 1 hour before cooking to reach room temperature. Trim all the fat off the meat. Using a sharp knife, pierce meat about 1/2-inch deep and insert slivers of garlic in each hole, pushing all the way in. Lightly spray the meat with olive oil and season generously with salt, pepper and rosemary. Place the thermometer all the way into the center of the meat.: When you leave the roast at room temperature for an hour the surface warms so it cooks more evenly, which reduces the gradient between crust and center. You will notice the meat losing its chill, and the fibers relax which allows seasonings and the inserted garlic to mingle more naturally with the flesh. As you poke the holes and slide in the slivers, listen for a soft tearing of the muscle, and feel the garlic settle in snug pockets. Press the garlic all the way so it does not fall out during roasting. A common mistake is inserting too large a sliver which can create a raw garlic center, or inserting them too shallow so they burn on the surface. The gentle spray of olive oil creates an even film that helps the dried chopped rosemary and cracked pepper adhere, promoting an even browned crust while preventing the seasonings from sliding off during transfer to the pan. Finally, ensure the meat thermometer reaches the thickest part, avoiding bone or fat pockets, so you get an accurate readout during roasting.
Preheat oven to 350 F. When the oven reaches 350 F, place the roast in a roasting pan and put in the oven.: You will sense the oven hit the set temperature when preheat indicators turn off or the light goes out. Placing the roast into a fully preheated oven ensures predictable searing and browning from the moment it goes in. As the roast slides into the pan you may hear a faint hiss as residual moisture meets the hot air, and the initial scent will be subtle, building into something much more savory as time goes on. Position the pan so air can circulate; avoid overcrowding. A typical misstep is putting the roast into a cold oven, which leads to longer cooking and a paler exterior. Aim to transfer confidently and close the oven quickly to retain heat.
Roast until the thermometer reads 130° for rare, 140° for medium rare, 150° for medium, and 155-160° for well done. Remove the roast from the oven and let it rest 10-20 minutes before you cut it so that the juices distributed evenly. I personally remove my roast beef from the oven when it is 135° for medium rare. The temperature will rise an additional 5 degrees as it sits.: As the roast cooks you will notice the exterior deepen in color and the kitchen fill with the sweet scent of roasted garlic and warm herbs. The thermometer is the most honest signal of progress, and pulling the roast a few degrees earlier accounts for residual carryover cooking. When you remove the roast and tent it loosely with foil, you will hear a gentle settling silence as internal juices redistribute, and after resting those juices will yield clean, slightly glossy slices rather than a flood on the cutting board. Avoid skipping rest time, as slicing too soon squeezes juices out and leaves the meat dry. Also, resist the urge to slice immediately to check doneness; patience pays off for texture and succulence.
Slice thin and serve.: After resting, you'll find the crust slightly firmer and the interior tender and rosy when cooked to medium rare. Use a sharp slicing knife and cut against the grain for the most tender mouthfeel. As you slice, you may notice pockets of softened, roasted garlic distributed through each piece, delivering concentrated flavor bursts. Arrange slices on a warm platter so they retain heat, and serve promptly. A common error is slicing too thickly, which masks the delicate interplay of crust and inner tenderness; aim for thin, confident strokes for the best experience.