Preheat oven to 350°. Mix rosemary and garlic in a small bowl. Sprinkle one side of roast lightly with salt and pepper. Spread with half the rosemary-garlic mixture, pressing it on. Drape half the prosciutto lengthwise over loin. Holding the prosciutto on the roast, turn it over. Sprinkle with more salt and pepper, spread remaining rosemary mixture over the top, and drape with remaining prosciutto. Tie roast at 1 1/2-in. intervals with heavy cotton string, tucking prosciutto into place.
Pour oil into a large frying pan over medium-high heat. Add roast and cook until prosciutto is crisp and beginning to brown on the bottom. Turn roast and cook until other side is crisp and browned, about 5 minutes total. Transfer to a plate.
Add fennel bulbs to frying pan and cook, stirring often, until beginning to brown, about 5 minutes. Spread in a large roasting pan. Add wine to frying pan and bring to a boil, stirring to scrape up browned bits. Pour over fennel. Set pork loin on top.
Bake until a thermometer inserted in the center of roast reaches 135° for medium, about 45 minutes. Transfer roast to a board and let rest in a warm place 15 minutes (temperature will rise to 140°).
Meanwhile, set roasting pan over 2 burners on high heat, and cook, stirring often, until liquid is almost evaporated, 5 to 10 minutes. Remove from heat and stir in olives and fennel fronds.
Cut string from roast, slice, and serve with fennel mixture.