Whisk together olive oil, red wine vinegar, onion powder, salt, pepper, jalapeno jelly and garlic in a mixing bowl until well combined. Set aside.
Take flat iron steak out of packaging and lay out on a cutting board. Using a meat mallet, tenderize a few times to ensure an even thickness throughout. *Note* This is not a tough piece of meat so you don’t need to beat it down, you just want to make sure it cooks evenly.Â
Pop the steak into a large plastic bag or tupper ware and pour on marinade.
Set on a plate and leave to marinade at room temperature for at least 30 minutes but no more than an hour.
About 10 minutes before cooking, preheat your grill over high heat and let it get nice an hot.Â
Remove steak from marinade, and grill over high heat with the lid closed for approximately 6 minutes.Â
Flip to the other side and grill with the lid closed over high heat for another 5-6 minutes.Â
Remove from grill and let rest for at 5-10 minutes.Â
Slice against the grain, top with chimichurri and enjoy!