Pat steaks dry with a paper towel and season generously with salt and pepper on both sides.
Add oil to a medium stainless steel skillet and heat on high. When the oil is hot add steaks and cook 4 to 4 1/2 minutes per side for medium-rare, flipping only once and getting a good sear on both sides.
Remove steaks to a plate and cover with foil.
Add balsamic vinegar, beef broth and rosemary to the same pan whisking to pick up any beefy bits left behind by the steak. Continue to cook over medium-high heat for about 4 minutes until sauce is reduced by at least half and can coat the back of a spoon. Remove rosemary sprig.
Turn heat to low and whisk in butter one chunk at a time, allowing the butter to be incorporated before adding more. Once all the butter is incorporated remove from heat and adjust seasoning to taste with salt and pepper. Serve over steak or on the side as a dipping sauce. Makes about 1/3 cup of sauce.
Cooking time for steaks will be dependant on size. The steaks I used were approximately 8 ounces each and 1 1/2 ” thick.