Place beef with cilantro marinade in 1-gallon resealable plastic bag. Squeeze out excess air and seal the bag. Marinate in the refrigerator for 1 hour 20 minutes.
Preheat grill to 250 to 300 degrees F (120 to 150 degrees C).
Combine onion, orange bell pepper, red bell pepper, yellow bell pepper, and margarine on a sheet of aluminum foil. Season with black pepper and seal vegetable packet.
Remove beef from the resealable bag. Place on a sheet of aluminum foil. Pour the remaining cilantro marinade on top. Seal the beef packet.
Place the vegetable and beef packets on the grill.
Grill vegetables until softened, 30 to 35 minutes. Grill beef until it begins to firm and is hot and slightly pink in the center, 30 to 40 minutes. An instant-read thermometer inserted into the center should read 140 degrees F (60 degrees C).
Grill flatbreads until warm and toasted, 5 to 8 minutes.
Arrange vegetables, beef, salsa, and sour cream on each of the flatbreads and serve.