Pat the steak dry thoroughly with paper toweling. Salt and pepper liberally. Cover with plastic wrap and rest at room temperature for 30 minutes.
When the steak is 5 minutes away from the end of its rest, set a cast-iron skillet over medium heat. After 2 minutes, add the butter and olive oil. When the butter is melted, tilt the pan to spread the butter and olive oil all around the skillet. Add the steak. Leave undisturbed until the bottom of the steak is nicely caramelized, about 5 minutes.
Flip the steak and cook until it reaches your desired level of doneness (see Recipe Notes #3 and #4). While the steak cooks, baste it frequently by spooning the butter/olive oil mixture in the skillet over the top of the steak.
Remove the steak to a cutting board and rest for at least 5 minutes and up to 10.
Meanwhile, cook the mushrooms and onions. Without wiping out the pan, add the mushrooms and onions. Saute over medium heat until the mushrooms have given up their liquid and the onions are softened, about 5 minutes. Add the Maggi seasoning (or coconut aminos), 1/4 tsp salt, and 1/2 tsp black pepper. Continue sauteing until the vegetables develop a bit of caramelization, about 5 additional minutes.
Slice the steak against its grain and serve with the mushrooms and onions.