Heat your oven to 350°F. Peel your garlic and finely chop it together with your rosemary. Set the mixture aside in a tiny bowl.

When your oven is ready, place your butter in a 9x13 pan or roaster and set the pan on the center oven rack for about 4 minutes, to melt the butter.

While the butter is melting, place your roast horizontally in front of you and use a sharp, serrated knife to cut vertical slits in the pork. Make the slits about four inches long, two inches deep and space them at least an inch apart. (See photo in post.)

Stuff your rosemary-garlic mixture into the slits you made in the roast. Wash your hands well.

When the butter is melted, take your pan out of the oven and place the pork in it. Season the roast with your salt and pepper. Pour the olive oil over the top.

Bake the pork initially for 30 minutes, uncovered. During that time, chop up your onion. After the pork has cooked for 30 minutes, add your chopped onion to the bottom of the pan.

Roast the pork for another 30-35 minutes, until the internal temperature reaches 160 degrees F.

Let the pork rest for 10 minutes before slicing it.