Preheat oven to 300 degrees. Place garlic on foil, drizzle with 1 Tbsp. oil. Wrap garlic and bake on a sheet pan until soft, about 45 minutes. Squeeze garlic flesh from the head into a small bowl. Divide in half.
Heat 1/3 cup olive oil in a skillet. Add prosciutto, lemon zest and pepper flakes cooking over medium heat until prosciutto begins to render but does not brown or crisp (3-5 minutes.) Add rosemary and cook another 30 seconds.
Strain mixture through a fine mesh strainer, reserving oil. Add half of the roasted garlic and 1 Tbsp. olive oil to prosciutto mixture and stir. Allow prosciutto mixture to cool slightly.
Position roast fat side up. Cut top 1/3 of roast horizontally, stopping 1/2 inch before the edge. Open top one-third flap. Starting from the center, keeping knife level with first cut, slice bottom 2/3 portion in half horizontally, stopping about 1/2 inch before the edge. Open this flap. Cover with plastic wrap and pound evenly. Season pounded pork with salt and pepper. Evenly spread inside of roast with the prosciutto-garlic mixture, leaving 1/4 inch border on all sides. Starting from the short end, Roll up pork completely and secure with butcher twine in 1-inch intervals.
Mix reserved cooking oil with remaining half of roasted garlic and stir. Set aside for serving.
Refrigerate roast covered for 1 hour. Bring roast out to sit at room temperature 15 minutes before cooking.
Heat 2 Tbsp. reserved oil in a skillet over medium-high heat. Carefully brown pork on all sides (around 6-8 minutes total). Preheat oven to 275. Set wire rack in a rimmed baking sheet and grease with cooking spray. Set roast fat side up on a prepared rack and cook for 1 1/2 to 2 hours, or until meat registers 135 degrees. Remove roast from oven and let rest for 20 minutes.
Whisk lemon juice, remaining half roasted garlic and reserved oil. Salt to taste.