In a large skillet, cook onion, sweet peppers, and garlic in hot oil over medium-high heat about 5 minutes or until crisp-tender. Add shredded beef, undrained tomatoes, jalapeno pepper (if using), vinegar, cumin, and pepper. Bring to boiling; reduce heat. Simmer, uncovered, about 5 minutes or until heated through. Serve over rice.
Tip: Because hot chili peppers contain volatile oils that can burn your skin and eyes, avoid direct contact with chiles as much as possible. When working with chile peppers, wear plastic or rubber gloves. If your bare hands do touch the chile peppers, wash your hands and fingernails well with soap and water when you are done.