Combine carrots, red peppers and onion in a 6-qt. pressure cooker. Top with roast. In a small bowl combine pineapple, soy sauce, brown sugar, vinegar, garlic and pepper; pour over roast. Lock lid; close pressure-release valve. Adjust to pressure-cook on high for 40 minutes. Let pressure release naturally. Press cancel. Remove roast from pressure cooker. Cool slightly; shred roast with 2 forks., Skim fat from cooking juices; return juices and vegetables to pressure cooker. In a small bowl, mix cornstarch and water until smooth; stir into pressure cooker. Select saute setting and adjust for low heat. Simmer, stirring constantly, until thickened, 1-2 minutes. Return beef to pressure cooker; heat through., Serve in lettuce leaves. If desired, sprinkle with green onions.