Pat the steak dry thoroughly with paper toweling. Cover with plastic wrap and rest at room temperature for 30 minutes. Liberally salt and pepper both sides of the steak right before grilling.
Meanwhile, prepare your charcoal grill. Light the coals using a chimney starter. When the coals are thoroughly ashed over, dump them into the kettle of your grill on one side for indirect heating. Place the grate on top and clean well.
Place the steak directly over the hot coals. Grill for 1–3 minutes, until the steak develops a nice sear. Flip and repeat on the other side.
Move the steak to the indirect side of the grill (but still close to the coals). Grill, flipping once, until done to your liking (see Recipe Notes #3–4).
When the steak is approximately 5 minutes away from being done, heat a cast-iron skillet over high heat on the stovetop. When the skillet is hot, reduce the heat to medium-high and add the butter. When the butter is melted, tilt the pan to spread the butter and olive oil all around the skillet. Add the onions and mushrooms. Saute, stirring frequently, until the vegetables are softening and turning a light, golden-brown color.
Remove the steak from the grill when it reaches your desired doneness. Rest for a minimum of 10 minutes. Slice the steak against its grain and serve with the mushrooms and onions.
If you are following a diabetic or low-carb diet, please note that the vidalia onion does contain some carbs, so keep that in mind and adjust how much you eat if you're near your daily carb limit.
If you are following a paleo, whole 30, or gluten-free diet, you can substitute coconut aminos for the Maggi seasoning sauce.
If your coals are not putting out sufficient heat, you may need to finish the steak directly over the coals. Or, move to the indirect side and cover the grill. Completely open the top and bottom vent.
To get a general idea of "where the steak is" in the cooking process, press the center of the meat with your index finger. Compare the "give" of the meat to what it feels like if you press on different parts of your face (a.k.a., the "face test"):
The hollow of your cheek (fleshy with no or very little resistance) = approximately rare to medium rare
Your chin (fleshy with some resistance) = approximately medium
Your forehead (firm with more resistance) = approximately well done.
If you decide to use an instant-read thermometer, insert into the thickest part of the meat. Doneness guidelines (take the meat off and let rest for 10 minutes):