Preheat broiler. Slice tenderloin into 1/2-inch-thick slices; flatten slightly with palm of hand. Arrange slices on unheated rack of broiler pan. Broil 4 inches from heat 4 to 6 minutes or until just slightly pink in center, turning once.
Meanwhile, halve, seed, and peel avocado; transfer to small bowl. Mash with fork. Stir in 1/4 tsp. salt and 1/8 tsp. pepper.
Spread bottoms of bread quarters with avocado mixture. Layer red onion, pork, and tomato on top of avocado mixture. Add bread tops; halve. Serves 4 (2 pieces each).