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 lb. meat lightly with the flat side of a meat mallet to about a 13x8-inch rectangle. (If necessary, use a portion of a second tenderloin to make 18 oz. Overlap and lb. 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 oven for 50-60 minutes or until meat is tender and slightly pink (160 degrees) 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.

To make Mustard Sauce, in small saucepan combine all ingredients. Cook over low heat for 2-3 minutes or just until heated through. Do not boil. Serve immediately with pork slices.

Yield: 6 servings