Set oven to 250 degrees and salt and pepper the steaks to your taste.
Put oil in a large pan on high flame. Once oil is hot, cook steaks for 2 minutes on each side.
Lower the flame to medium-high and add in half the butter, onion powder, garlic and rosemary. Cook steaks for the level of rareness that you want, while keep spooning the sauce onto the steaks. A minimum of 15 minutes for the most rare.
Grease a baking pan and put the steaks in the oven to keep warm while dealing with the rest of the sauce. Pour the leftover juice in pan into a bowl, leaving about a 1/4 cup inside. Raise the heat to high and pour in wine and brandy. Mix well.
Pour in chicken broth and boil for about 10-12 minutes. Add in the other half of butter and Dijon mustard. Whisk well.
Flavor with the amount of salt and pepper you wish. Serve with the sauce poured over steaks.