For the roast: Remove the pork loin from the fridge 30 minutes before cooking. Preheat the oven to 400 degrees F.
Place the rosemary, garlic and fennel seeds in a small pile on a cutting board. Run your knife through the pile a few times to chop everything finely and mix the ingredients together. Place the herb mixture in a small bowl and stir in the salt and olive oil to make a paste. Set aside.
Using a sharp paring knife, make ten 1-inch-deep incisions in the top and sides of the pork loin. Rub the herb paste all over the roast, especially in the cut surfaces. Tie the roast in three spots using butcher's twine. Transfer to a rimmed baking sheet.
Roast to an internal temperature of 145 degrees F, about 40 minutes. Remove the roast from the oven and allow to rest for 10 minutes before slicing.
For the gremolata: While the roast rests, whisk together the parsley, lemon zest, lemon juice, salt, pepper and olive oil in a small bowl.
Serve the gremolata alongside the sliced pork.