Preheat an oven to 350 F. Evenly salt the sirloin steak on both sides with ½ teaspoon kosher salt.
Step 2
Prepare a medium sized nonstick, ovenproof saute pan over high heat until hot. Add the avocado oil and swirl it around in the pan. Add the steak and turn the heat down to medium-high. Sear the steak on the first side for 3-5 minutes. Flip the steak and sear it on the other side until nicely browned. Remove to a plate.
Step 3
Add the butter and then the mushrooms which have been chopped into quarters and saute them until brown about 3-5 minutes on medium-high heat.
Step 4
Add ¼ teaspoon kosher salt and balsamic vinegar to the mushrooms and stir well.
Step 5
Push the mushrooms to the side and add the steak back into the pan. Place the entire pan in the oven and bake for 10 minutes or until the steak is medium-rare. Remove from the oven and allow the steak to rest on a cutting board for 5 minutes. Slice with a sharp knife and top with mushrooms.