Whisk together the olive oil, vinegar, garlic, mustard, herbs, sea salt, and black pepper in a small bowl.
Place the flank steak in a large resealable bag, then pour in the marinade. Squeeze the air out of the bag, seal tightly, then turn the bag several times to evenly distribute the marinade. Refrigerate for 2-4 hours.
Remove the steak from the bag, discarding the marinade, and pat it dry. Allow the steak to stand at room temperature for 20-30 minutes.
Preheat the grill to high heat and lightly oil the grate. Grill the flank steak for 5-6 minutes per side until desired doneness is reached (135°F for medium rare).
Allow the steak to rest for 10 minutes, then slice it against the grain into thin strips before serving.