1. In a small bowl, whisk together soy sauce and brown sugar until dissolved; set aside. Cut tenderloin crosswise into 1/4-inch-thick slices. Trim off any fat or connective tissue. Place 1 slice between 2 pieces of plastic wrap. Using the flat side of a meat mallet, evenly pound out the slice, keeping it roughly in a rectangular shape, until it is 1/8 inch thick. Do not overpound slices or they will begin to disintegrate. Remove plastic wrap, and transfer to a large plate. Repeat process with remaining meat.

2. Dip 1 slice beef in the soy sauce mixture, and place it on a clean surface. Sprinkle with salt and pepper. Place 2 pieces of scallion green and 1 piece of each pepper across the shorter length of the beef, so that the vegetables extend over both edges by about 3/4-inch. Roll up lengthwise, and skewer near the edges with 2 toothpicks. Set aside. Repeat with remaining beef and vegetables.

3. Lightly brush a grill pan with olive oil, and heat until hot, or heat the oven broiler. Grill or broil beef rolls, brushing with sauce and turning, until medium rare, 2 to 4 minutes. Cut each roll in half, or cut any longer rolls into 3 pieces. In a small saucepan bring remaining sauce to a boil for 3 to 5 minutes. Pour into a small bowl, and add scallion rings. Transfer beef rolls to a serving platter, and serve with sauce on the side.