Combine 1 tablespoon of olive oil, balsamic vinegar, rosemary, garlic and salt in a large plastic bag.
Add steak, evenly distribute marinade and seal tightly.
Marinate for three or more hours in the refrigerator.
Preheat grill on high heat.
Grill steak, untouched, on high heat for 4-5 minutes.
Flip steak and continue cooking to desired doneness. Remove from heat and let rest for five minutes.
For vinaigrette, combine capers, shallots, parsley, red wine vinegar, remaining olive oil, lemon juice and lemon zest.
Thinly slice steak, top with caper vinaigrette and serve immediately.