1. Combine first 5 ingredients and 1/4 teaspoon salt in a small bowl, stirring with a whisk; pour into a large zip-top plastic bag. Add steak, bell pepper, and onion; seal bag, and marinate in refrigerator at least 6 hours, turning bag occasionally.
2. Preheat oven to 450°.
3. Transfer steak mixture to a heavy-duty foil bag that has been coated with cooking spray. Roll up bag tightly. Place bag on a jelly-roll pan or baking sheet. Bake at 450° for 40 minutes.
4. To serve, carefully cut a slit in bag. Sprinkle steak mixture evenly with remaining 1/4 teaspoon salt and cilantro; toss well. Divide mixture evenly among flour tortillas. Serve with fat-free sour cream, if desired.