Lay loin fat side down. Make a cut lengthwise down the center of the meat and about 2/3 of the way through it. Push meat flat and, starting from the deepest part of the center cut, make a 1-inch horizontal cut toward each side running the length of the meat. Push meat flat again.
Trim stem ends from mustard greens, rinse leaves, and dip in boiling water to wilt; drain briefly.
Line meat lengthwise with greens, overlapping as needed, extending about 4 inches beyond ends of meat. Spoon Boursin onto leaves down center of roast, then sprinkle evenly with parmesan. Fold extended leaves over cheese flush with ends of meat. Then lap leaves across cheese lengthwise. Gently roll loin back into a log (don't squish out cheese) and tie with cotton string at 2-inch intervals.
Set loin cut side down, in a 10- by 15-inch pan and sprinkle generously with pepper
Bake in a 375º oven until meat is 150º to 155º in center of thickest part (not in cheese), 50 to 55 minutes. Remove from oven and let stand at least 20 minutes.
Transfer roast to a platter and keep warm. Skim and discard fat from pan drippings. Add broth and whisk until smooth. Boil over high heat until reduced to about 1 1/4 cups, about 10 minutes. For a smooth sauce, whirl in a blender.
Cut loin into 3/4- to 1-inch slices, remove string, and serve with sauce.