2 tablespoons olive oil
3 cloves garlic, minced
3 tablespoons finely chopped fresh rosemary leaves
1 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
1 (2- to 2 1/2-pound) boneless pork loin roast, trimmed of excess fat (keep a thin layer of fat)
1 pound small red potatoes, halved or quartered if large
Arrange a rack in the middle of the oven and heat to 375°F.
Place the oil, garlic, rosemary, salt, and pepper in a small bowl and rub together with your fingers to create a paste. Rub the paste all over the pork, then place fat-side down on a rimmed baking sheet. Roast for 30 minutes.
Flip the pork to be fat-side up. Add the potatoes to the baking sheet, stir to coat with the rendered fat, and arrange to be cut-side down in a single layer around the pork. Continue roasting until the internal temperature of the pork registers 145°F, about 20 minutes more. Let the pork rest for 10 minutes before slicing and serving with the potatoes.