For Sauce: Heat oil in large skillet over medium-high heat until hot, about 3 minutes. Add shallots and garlic. Stir occasionally until golden brown.
Add beef broth, bring to boil and cook until half of the liquid is dissolved, about 5-10 minutes. Set mixture aside to cool for 10 minutes.
In blender or food processor, puree mixture with tarragon leaves, mustard and salt. Set aside in small bowl.
For Steaks: Bring steaks to room temperature for no longer than 30 minutes. Season with salt and pepper.