Allow steaks to come to room temperature for about 30 minutes prior to beginning to cook them.
Prior to beginning to cook steaks, preheat the oven to 400º F.
Pat the steaks dry with paper towels. Generously season the filet mignon steaks on both sides with Kosher salt and black pepper. America’s Test Kitchen says you can never over-salt or pepper a steak – and since I started believing that, my steaks have come out so much better! The coating on the outside flavors the whole steak, and these are thick.
Heat a skillet over medium-high heat and add butter and avocado oil. The avocado oil stops the butter from burning at a high temperature. Add the steaks to the hot pan, and cook them for 3-4 minutes per side for a good sear. Both sides should be cooked the same amount of time to ensure even cooking inside.
Add garlic cloves and rosemary to the skillet, infusing the butter with flavor, and then spoon the butter mixture over the steaks to coat them in the flavor. This is referred to as butter basting the steaks.
Add the skillet to the preheated oven and bake the steaks to the desired temperature. Thicker steaks will take longer and thinner steaks should be checked prior to adding them to the oven. See chart below for cooking temperatures, and check steaks with a meat thermometer every 2-3 minutes.
Once steaks have come to the desired temperature, remove them from the pan and allow them to rest on a plate for 10 minutes before serving.
Serve steaks with an extra spoonful of the pan drippings, or make a delicious 4-ingredient Creamy Black Peppercorn Sauce.