Heat oil in a large nonstick skillet over medium-high heat. Add onion, bell peppers, and garlic; sauté 4 minutes or until golden brown. Reduce heat, and cook 8 minutes or until tender, stirring occasionally. Stir in sugar, oregano, vinegar, and salt. Add cheese; cook 2 minutes or until cheese melts, stirring constantly.
Combine broth and beef in a microwave-safe bowl. Microwave at high 1 minute or until hot.
Place 3 ounces beef mixture in each bun; top each with 1/2 cup cheese mixture. Serve immediately.