Heat oil in small skillet. Cook peppers, onions, and garlic until vegetables are tender. Add vinegar, sugar, oregano, and cheese. Stir mixture and turn heat to simmer to let cheese slowly melt, stirring periodically. Once cheese is melted, remove from heat and set aside until ready to assemble sandwich.
Meanwhile, spray cut sides of roll with butter spray. Place buttered side down on a hot griddle and toast buns until warmed and start to brown.
Place roast beef and beef broth in microwave safe bowl. Microwave in two 30-second intervals. Assemble sandwich - using a fork remove beef from broth and layer on bottom half of bun. Top with cheese pepper mixture and then cover with top bun half. Enjoy!