Combine the oil and rest of the ingredients in a food processor or blender.

Pulse until the peppers and cilantro are smooth.

Place marinade in a large ziploc bag.

Add large flank steak and mix the marinade around to completely coat the steak.

Marinade for at least 8 hours, but up to 24.

Grill large steak for 5-10 minutes per side (5 minutes for rare meat, 10 for well done).

While steak is grilling, add marinade to a small saucepan.

Bring to a boil and let simmer for 8-10 minutes.

Cut the steak across the grain into thin diagonal slices.

Serve steak with the marinade sauce.