Preheat the oven to 500°F. Remove the beef from the refrigerator and let it sit at room temperature while the oven preheats.
Mince the garlic, then sprinkle it with the salt and use the side of your knife to mash it into a paste on the cutting board (if you have a mortar of pestle, that also works). Add the rosemary and press it a few more times to mix. Transfer the garlic mixture to a small bowl, then add the olive oil and pepper and stir to combine. Rub the seasoning all over the roast beef, then put it in a roasting pan that will hold it comfortably.
If you have a thermometer probe or a thermometer you'll be able to see and read through the oven door, insert it now.
When the oven is ready, roast the beef for 15 minutes (5 minutes per pound, adjust the time if your roast weighs more or less). After 15 minutes, turn off the oven but do not open it!
If you're able to get a temperature reading as it cooks: Let the beef sit in the oven, undisturbed until it reaches 135°F for medium-rare (145- 150°F for medium), there will be little to no carry-over cooking once you remove it), this took about 45 minutes (in my oven) but it can take up to 1 1/2 hours, depending on how well your oven retains heat.
Remove the roast from the oven and let it rest for 10 minutes before you cut it so that the juices distribute evenly. Slice thin and serve.
Notes
If you don’t have a meat probe and need to open the oven to check the temperature with an instant read thermometer: Let the beef sit in the oven, undisturbed, for 1 hour. After one hour, use an instant-read thermometer to check the temperature. If it's 138-140°F (148-150°F for medium), remove it from the oven. If not, return the oven to 300°F and check often until the meat reaches 135°F (because it was removed from a hot oven, the roast will raise another 5°F as it rests). Remove the roast from the oven and let it rest for 10 minutes before you cut it so that the juices distribute evenly. Slice thin and serve.
For a larger roast, adjust the salt, pepper, garlic, etc and the cook time as noted above.