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1
Place the bread in a food processor fitted with a blade attachment and pulse into fine crumbs, about 15 (1-second) pulses.
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2
Remove to a large bowl and set aside.
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3
Place the fennel, apple, and onion in the food processor and pulse into fine pieces no larger than a grain of rice, scraping down the sides of the bowl as needed, about 20 (1-second) pulses; set aside.
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4
Melt the butter in a large frying pan over medium heat.
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5
Add the processed fennel mixture and the rosemary, season with salt and pepper, and cook, stirring occasionally, until the vegetables are softened, about 15 minutes.
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6
Remove the mixture to the bowl with the breadcrumbs and stir until combined and the bread is evenly moistened.
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7
Taste and season with salt and pepper as needed; set aside to cool for at least 15 minutes.
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8
Meanwhile, line a rimmed baking sheet with foil and cut 6 (18-inch) lengths of kitchen twine; set the baking sheet and twine aside.
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9
Mix the salt, fennel seeds, rosemary, pepper, and sugar in a small bowl until combined; set aside.
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10
To butterfly the pork, place it on a cutting board with one end pointing toward you.
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11
Slice lengthwise down the center, almost but not quite cutting through the pork, leaving about 1/2- to 3/4-inch thickness of meat intact.
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12
Open the pork up like a book and push on it to flatten.
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13
Starting on the left side, with the blade of the knife parallel to the cutting board and the blade facing left, slice down the length of the seam, maintaining the 1/2- to 3/4-inch thickness.
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14
Pull the meat open and press down to flatten.
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15
Continue cutting and flattening until the entire left half is 1/2- to 3/4-inch thickness.
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16
Rotate the pork and repeat on the other half.
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17
Evenly spread the reserved stuffing over the pork, leaving a 1-1/2-inch border.
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18
Horizontally arrange the prosciutto over the stuffing, one piece slightly overlapping the next.
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19
Starting on the right side, roll the pork into a tight cylinder.
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20
Tie the pieces of twine around the pork, spacing them about 1 1/2 inches apart and trimming off any excess.
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21
Using your hands, rub the oil evenly over the outside of the pork, then rub the the roast with all of the reserved spice mixture.
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22
Place the roast seam-side down on the prepared baking sheet and let it sit at room temperature for 30 minutes.
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23
Meanwhile, heat the oven to 450 degrees F and arrange a rack in the middle.
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24
Roast until the pork is light golden brown on top, about 30 minutes.
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25
Reduce the oven temperature to 375 degrees F, rotate the baking sheet, and roast until an instant-read thermometer inserted into the center of the pork registers 145 degrees F, about 25 to 35 minutes more.
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26
Let the roast rest on the baking sheet loosely tented with foil for 20 minutes before slicing.
