In a small bowl, combine the butter, lime zest and juices, seasonings, garlic and cilantro. Mix well. This can be made up to a day in advance so the flavors can combine.
Store the butter in an airtight container in the refrigerator.
Prepare the steak:
Take the steak out of the fridge. Dust both sides of the steak with salt. Place the steak back in the fridge until the grill is ready.
Grill the steak:
Heat the grill to a high heat. Aim for about 450-500º Fahrenheit.
Place the steak on the hot grill and press lightly to get sear marks. Flat iron steaks grill quickly at a high heat. You’ll only need about 5 minutes per side to get the internal temperature to 140º Fahrenheit for medium rare. (This is dependent on the thickness of the steak so be sure to measure the internal temperature. Remember that the temperature will continue to rise after you take the steak off the grill.)
Remove the steak from the grill and place it on a plate.
Slice or scoop the chili lime butter and place it on top of the steak. Allow the steak to rest for 5 minutes. Brush the butter over the steak if needed to get flavor to every part of the steak.
Cut the steak against the grain for the most tender slices.
The calories shown are based on the recipe serving 6, with 1 serving being 1/6 of the steak. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information. The information in the nutrition box are calculated through a program and there is room for error. If you need an accurate count, I recommend running the ingredients through your favorite nutrition calculator.**