How to Make It
Cook rabe in boiling water 3 minutes; plunge into an ice bath for 1 minute. Drain well. Wrap rabe in paper towels; squeeze dry. Chop rabe into 1 1/2-inch pieces. Place in bowl with 1 tablespoon oil, 1/4 teaspoon salt, pepper, and garlic; stir well.
Preheat oven to 325°F.
Cut into pork loin lengthwise from right to left, 3/4 inch from bottom, keeping knife parallel with cutting board; do not cut through the other side. Continue slicing lengthwise from right to left, unrolling loin as you slice, to form a 3/4-inch-thick rectangle. Season with remaining 3/4 teaspoon salt.
Arrange prosciutto in layers to cover inside of loin. Spread rabe mixture on top, leaving a 1-inch border. Roll pork up left to right. Tie with twine in surgeon's knots at 2-inch intervals.
Combine stock, wine, and Marmite in a roasting pan. Place pan over medium heat; cook until Marmite dissolves, stirring. Set a roasting rack in pan.
Heat a large skillet over medium-high. Add remaining 2 tablespoons oil. Place loin in pan; cook 12 minutes or until browned. Place loin on rack; cover loosely with foil. Roast at 325°F for 50 minutes or until meat registers 150°F. Remove pork from pan; let pork stand 20 minutes. Swirl butter into pan juices until butter melts. Cut pork into 3/4-inch slices; serve with jus.