Trim any fat from meat. Using a sharp knife, make a lengthwise cut down the center of roast, cutting to, but not through, the other side. Spread the meat flat. Place tenderloin between 2 sheets of plastic wrap and pound meat lightly with the flat side of a meat mallet to about a 13×8-inch rectangle. (If necessary, use a portion of a second tenderloin to make 18 ounces. Overlap and pound the pieces to make a single rectangle.) Fold in the narrow ends as necessary to make an even rectangle. Spread mustard evenly over tenderloin.
Stir together romaine, herbs, bread crumbs, and egg white in a medium bowl. Spoon evenly over pork. Roll tenderloin up jelly-roll style, beginning at narrow end. Tie meat with string, first at center, then at 1-inch intervals.
Place meat on rack in a shallow roasting pan. Brush oil over meat. Sprinkle with pepper. Roast, uncovered, in a 375 degree F oven for 50 to 60 minutes or until meat is tender and slightly pink (160 degrees F) and juices run clear. Transfer to a warm platter. Remove strings; keep warm while preparing sauce. To serve, cut tenderloin into 12 slices. Spoon Mustard Sauce over each serving. Sprinkle with chives if desired. Makes 6 servings.
Mustard Sauce: In small saucepan combine 1/3 cup plain fat-free yogurt, 2 tablespoons fat-free mayonnaise or salad dressing, 1-1/2 to 2 teaspoons Dijon style mustard, and 1 teaspoon honey. Cook over low heat for 2 to 3 minutes or just until heated through. Do not boil. Serve immediately with pork slices.