1. Preheat oven to 250 degrees Fahrenheit. Sprinkle steaks on all sides with salt and pepper.
2. Heat oil in a large skillet over high heat. Add steaks and sear until brown, about two minutes per side. Reduce heat to medium-low.
3. Add three tablespoons margarine, garlic, thyme, and rosemary to skillet. Cook steaks to desired doneness, turning occasionally and basting with pan juices, about 15 to 20 minutes. Transfer steaks to small baking pan and keep warm in oven.
4. Pour contents of skillet into small bowl. Return three tablespoons drippings to skillet and place over high heat. Add shallots and sauté two minutes. Add cognac and wine; stir. Add broth and boil until sauce is reduced to one cup, about 12 minutes.
5. Whisk in Dijon mustard; add remaining three tablespoons margarine, one tablespoon at a time. Season sauce to taste with salt and pepper.
6. Arrange steaks on plates. Whisk any accumulated juices from baking pan into sauce. Spoon sauce over steaks and serve.