Preheat your oven to 250 C/ 500 F on broil mode. Finely slice the onion into rings and place into a cast iron skillet or casserole dish. Arrange them slightly larger than the steak you’re using.
Massage the beef tenderloin with olive oil and season generously with salt and pepper. Position it over the onion rings. The onions will serve to absorb all the juices dripping from the steak.
Broil the steak for 5-7 minutes, then flip it and broil it for another 5-7 minutes. Lower the oven’s temperature to 200C/400F and finish broiling it for 10-12 minutes. Use a food thermometer to check the meat’s internal temperature; If you prefer medium-rare steak, then your thermometer should read 125 F/ 51 C, and if you prefer medium doneness, then 135F/57 C is what you should be aiming at.
Take the skillet out of the oven and transfer the tenderloin to a plate or cutting board. Wrap it in aluminum foil and let it rest for 10 minutes. Then, using a sharp knife, slice it into ½ inch (1.25 cm) pieces and transfer them to a serving dish.
To prepare the garlic and herb butter, melt and add the butter to a small bowl. Mince the garlic and finely chop the parsley and place them in the bowl along with the soy sauce, sriracha, and crushed red pepper. Collect any unburnt onions from the skillet, add them to the bowl with the rest of the ingredients, and whisk to combine.
Taste the garlic herb butter and add more salt if desired. Drizzle it on the sliced steak, and brush it to distribute evenly. Serve immediately!