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.
Set clean, well-oiled grill to medium-high heat. When grill is hot, using tongs, place steaks on grill and cook with grill lid open. Cook until marked and browned on both sides, about 3-5 minutes per side or until desired doneness, to internal temperature of at least 145F. To create a thick crust, cook first side of steak for an additional 1-2 minutes before flipping.
Remove steak from grill, place on serving platter and let rest for 10-15 minutes.
With a spoon, add an equal layer of sauce over the top of each steak.