1Between pieces of plastic wrap, place 1 pork tenderloin piece, cut side down. Gently pound with meat mallet or rolling pin, beginning in center, until 1/4 inch thick. Repeat with remaining pork pieces.
2In shallow dish or pie plate, stir Bisquick mix and mustard until mixed. In another shallow dish or pie plate, beat egg. Dip pork pieces in egg, then coat in Bisquick mixture.
3In 12-inch nonstick skillet, heat 2 tablespoons of the oil over medium heat. Add pork. Cook 6 to 8 minutes, turning once and adding remaining 1 tablespoon oil if needed, until golden brown on outside and no longer pink in center.
4Place pork on bottom halves of buns. Top with lettuce and tomato. Spread cut sides of bun tops with mayonnaise; add to sandwiches.