Serve this herbal and lightly mustard-infused roast as the centerpiece for a special occasion or Sunday dinner. On the side, potatoes and a green vegetable— like shaved asparagus or Brussels sprouts salad, perhaps. You can also enjoy this with Ham & Smoked Gouda Biscuits with Maple Butter and Ham Stacks Au Gratin with Thyme.
- Preheat the oven to 350 degrees F.
- Make the spice paste: Use a mortar and pestle or spice grinder to coarsely grind the mustard seeds and fennel seeds. Transfer to the bowl of a food processor; add the rosemary, oil, garlic, salt and zest of 1 of the lemons, and pulse to make a coarse paste.
- Arrange the pork on a work surface, fat side up, and cut it lengthwise along one side, opening it like a book, with the fat side as the “cover” of the book. Spread the spice paste inside, then close. Arrange 4 prosciutto slices across the top of the pork, slightly overlapping. Carefully turn the pork over and arrange the remaining 4 prosciutto slices across the bottom of the pork. Tie the roast with kitchen twine at 1 1/2- to 2-inch intervals. Place in a 9x13-inch roasting pan with a rack, fat side up, and roast 1 to 1 1/4 hours, or until internal temperature is 145 degrees F. (medium rare) to 160 degrees F. (medium) on a digital meat thermometer.