Heat your oven to 350°F. Peel the garlic and finely chop it together with the rosemary. Set the mixture aside in a tiny bowl.
When your oven is ready, place the butter in a 9x13 pan or roaster and set the pan on the center oven rack for about 4 minutes, to allow the butter to melt.
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 the 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. Tilt the pan back and forth a bit so the melted butter coats the whole bottom. Set the pork on top of the melted butter in the pan. Season the roast with the salt and pepper. Pour the olive oil over the top.
Bake the pork initially for 30 minutes, uncovered. During that time, chop up half an onion. After the pork has cooked for 30 minutes, remove the pan from the oven. Add the chopped onion to the bottom of the pan, around the pork.
Make sure to wear gloves/mitts again as you put the pork back in the oven to roast for another 30-35 minutes, until the internal temperature reaches 160 degrees F.
Let the pork rest for 10 minutes before slicing it. Use a large knife with a smooth blade to slice the meat. To serve, spoon the juices and onions on top of the sliced pork.
Store leftover pork covered in the refrigerator for up to four days. You should be able to freeze the sliced pork for up to three months.
The cooking time is for a 3-pound roast. If yours is smaller, it will cook faster.
When slicing a roast, don't use a serrated knife, because it will tear up the meat as you slice it. Use a smooth, chef's knife(affiliate link).