How to Make It
Stir together rosemary, garlic, 2 tablespoons of the salt, and 1 1/2 tablespoons of the pepper in a small bowl. Using a retractable-blade knife, make deep cuts 1/4 inch apart through skin and fat of roast. Rub rosemary mixture into cuts in roast and any other openings in the skin. Sprinkle roast with 1 tablespoon of the salt and 2 teaspoons of the pepper. Place roast, skin side up, on a wire rack in a lightly greased roasting pan. Let stand until pork is at room temperature, at least 2 hours.
Preheat oven to 350°F. Pour 2 to 3 cups water into roasting pan to cover the bottom of the pan. Bake in preheated oven on lowest oven rack until a meat thermometer inserted in thickest portion registers 175°F, 4 hours to 4 hours and 15 minutes
Meanwhile, combine port, honey, cloves, 2 tablespoons of the vinegar, and remaining 1/4 teaspoon each salt and pepper in a medium saucepan over high. Cook, stirring occasionally, until mixture thickens and reduces to a scant 1 cup, 15 to 20 minutes.
Remove roast from oven, and brush with half of the port mixture. Return to oven, and bake 10 minutes. Brush with remaining glaze, and bake until skin hardens, about 10 minutes. Remove roast from oven; let stand 20 minutes. Transfer roast to a serving platter. Pour pan drippings into a heatproof measuring cup; skim fat from the top. Combine pan drippings and remaining 1 tablespoon vinegar in a small saucepan over medium-high, stirring until warmed through, about 2 minutes.
Stir together mayonnaise and Dijon mustard. Beat heavy cream with an electric mixer on high speed until peaks begin to form. Fold in mayonnaise mixture until smooth. Serve immediately, or cover and chill up to 4 hours.
Before slicing roast, remove hardened, crispy skin from the top of the roast; arrange around roast on the platter. Serve with pan sauce and Whipped Dijon Cream.