Butterfly the pork loin and pound it out to an even thickness of 1/2 to 3/4 inch.
Add the broccoli rabe to a large pot of boiling water and cook until just tender, 2 to 3 minutes. Submerge the broccoli rabe in a bowl of ice water until cooled, then wring it out to remove as much water as possible. Roughly chop and add to a medium bowl.
To the broccoli rabe, add the breadcrumbs, sausage, Parmesan and egg; stir to combine. Lay a large piece of caul fat out on a large, flat work surface. Lay about 6 prosciutto slices in the middle, slightly overlapping. Arrange the flattened out pork loin on top of the prosciutto. Set the sausage mixture on the pork loin and shape it into a neat, even log. Wrap 2 pieces of prosciutto around each end of the log and fold the prosciutto that's underneath up and around the pork. Roll the pork loin around the filling and into a log, wrapping the caul fat around it in an even layer. Tie with butcher's twine to secure.
Coat a pan with olive oil and place it over medium-high heat. Sear the pork roulade until golden brown on all sides. Transfer to a baking dish and bake 10 minutes (reserve the fat in the pan for the Roasted Red Potatoes). Flip the roulade and continue to bake until the internal temperature registers 150 degrees F, about 10 minutes more. Let rest for 5 minutes, then remove the string. Slice and serve with Roasted Red Potatoes.
Roasted Red Potatoes:
Preheat the oven to 375 degrees F.
Put the potatoes in a large bowl along with the fat from searing the roulade. Add the Parmesan, garlic, rosemary and some salt; toss to coat. Spread the potatoes out on a baking sheet and bake until tender, 20 to 25 minutes.